Life histories of tree seeds

Reproduction of the tree is by no means complete with the development

of the new plant in the form of a ripened seed. The adjustments of the

various types of seeds to the environments in which the trees grow, and the

fate of the seed between the time of dispersal from the mother tree and its

germination and establishment as a growing tree are subjects, which merit

mention here.

The simpliest life histories are found in spring-ripening seeds such as

aspen, willow and silver maple. Immediately the seeds come to rest, they

may begin to germinate, since they are not dormant, and depend for successful

establishment on gaining a foothold before the rank vegetation of

summer causes too much competition. Species in this group require for the

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most part bare soil and a moist seed bed. They have no need of large food

reserves, or capacity to remain dormant — although red maple and elm

may become dormant upon drying and lie over until the next year, thus escaping

an unfavorable period. Spring-ripening seeds and those holding

seed in the cone until spring are relatively little sought after by birds and

animals, since other food is plentiful in spring.

Among the seeds which ripen in the fall, some exhibit more or less

pronounced dormancy, which protects them from fall germination and

freezing; others as in Pinus banksiana are retained indefinitely in the cone,

and shed only with the hot spring sun, or when the heat of a forest fire

opens the cones after many years. Where spring droughts are frequent and

winters mild, some species, as longleaf pine (P. palustris) and many of the

oaks (Ouercus), germinate in the fall, and extend their radicles during the

winter so that the seedlings have a firm foothold when the dry season arrives.

Many seeds ripening in late summer and fall are incapable of immediate

germination even when they fall on a moist, warm seedbed until they

have undergone a period of after-ripening, just as buds of trees, formed in

the late summer, do not open in the fall, although the weather may be favorable,

but require exposure to the cold of winter before attaining a condition

of readiness to unfold. In both cases the dormancy is of great benefit,

protecting the new growth from certain freezing. Fall-ripening seeds

may also be held on the tree until danger of fall-sprouting is past; then

winter cold prepares the dormant embryo for prompt spring germination.

Few if any species with permeable seed coats lie thus more than one winter.

Some species of ash like Fraxinus excelsior and F. nigra have an immature

embryo when shed from the tree. The embryo elongates during the

fall and after-ripening proceeds during the winter. Large-seeded species,

such as oaks and walnuts, which are injured by drying find ideal conditions

for after-ripening, protection from drought and frost when buried

(and forgotten) by squirrels.

Vivipary, rare in tree seeds, is exemplified in the mangrove. Here the

embryo germinates while in the ovary on the parent tree. The seedling may

reach a foot or more in height before dropping from the tree.

Fleshy fruits, a favorite food of some birds, are often associated with

hard seeds, fortified by a dense or bony seedcoat. This bony layer is frequently

a part of the pericarp, e. g. Cornus, Crataegus, Prunus. These seeds

are apt to be eaten by birds and pass through the alimentary tract of the

bird not only unharmed, but actually rendered permeable and ready for

germination by the acids in the digestive fluids. Seedlings of Juniperus and

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Prunus are abundant along fence rows frequented by birds, but often scarce

in the vicinity of the parent trees. The impermeable covering may also

serve to protect these seeds from the acids and alcohols present in decaying

fruits. Not infrequently these seeds require prolonged after-ripening as

well. In nature these drupes with stony endocarps are eroded by fungi and

broken by frost action so that germination of the seed occurs the second

spring following ripening. They often lie several years longer, when heat

and moisture are insufficient. After-ripening at low temperatures is required

most commonly in seeds from northern and cold regions. Dormancy

is a valuable aid in preventing too early germination, or germination

in the fruit. Seedcoats impermeable tо water or oxygen favor perpetuation

of some species, by aiding longevity, and when combined with a

dormant embryo insure germination at a favorable season.

THE TESTING OF GERMINATION

The Purpose of Germination Tests: — Like other estimates of large

numbers of objects, seed tests must be made by sampling, examining a

small percentage of the material, since in germination tests especially, the

seeds tested are usually not saved for use in producing plants. The purpose

of germination tests must be to determine the proportion of the seeds in a

given lot which are capable of germinating.

Since it is the capacity for germination which must be investigated, all

other variable factors should be eliminated as far as possible. The statement

is frequently made that the conditions for germination tests sliould

approximate those to which the seeds will be exposed in the nursery or

field; then a truer picture of results to be attained in practice may be

gained, it is claimed. While this might at first appear a plausible argument,

it is unfortunately very difficult to make such tests, and the variable

weather and soil conditions make them extremely uncertain. Seed of excellent

quality may fail to germinate completely under unfavorable field conditions.

Such tests are also very time-consuming and require elaborate

greenhouses for testing during the winter. Where empirical data on the relation

between laboratory tests and field results are lacking, field tests are

highly desirable. These will be discussed under "plant per cent". Severe

tests of germination under controlled conditions (sprouting power, etc.) are

also possible. The only true basis for germination tests, which combines

uniformity of results and completion of tests in the shortest time, is the determination

of the percentage of seed which germinates under optimal

conditions for the species in a given time.

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Treatments preparatory to germination tests: — Many seeds fail to respond

promptly when placed in an environment favorable to germination,

unless they have first experienced a period of conditioning, preparing the

seed by chemical changes, embryo maturity, or by influences which render

the seed coat permeable to water and oxygen. Only when a "readiness to

germinate" has been attained by appropriate pre-treatment it is worth while

to conduct germination tests. It is true that there is more or less delay in

germination in all species, and that some absorb water more promptly than

others; some after-ripening takes place in the germinator. The stimulus of

spring imparted by the warmth and moisture of the germination medium

affects some seeds sooner than others. Dormancy is relative, but pretreatment,

when used, should be standardized for each species, if different

tests are to be comparable.

Soaking: — The simplest way of hastening moisture absorption is by

immersing the seeds in water. The efficacy of this treatment will vary with

each species. Soaking, especially prolonged soaking, has often been found

injurious. If employed at all, soaking should be of short duration, — and

the time accurately measured and standardized. Requiring as it does another

manipulation, complicating seed-testing, it is not recommended except

in special cases. Satisfactory results can usually be obtained without

recourse to soaking and some lots of seed may be especially sensitive to

and damaged by such treatment. However, some seeds may be "conditioned"

to tolerate submergence by placing on moist blocks of porous fired

clay.

Stratification: — Species which regularly possess dormant embryos,

requiring afterripening, should be stratified under standard conditions before

a germination test is attempted. Since stratification usually denotes

mixing with peat or sand and the separation of the seed from this material

is often time-consuming, the author has suggested setting up the seed on a

portable germinator which can be placed in a cold chamber for the appropriate

stratification period and then moving to a temperature favorable for

germination.

Disinfection: — The need for seed treatment before germination tests

to reduce fungous attack varies greatly with the species age of seed and

length of test. If the germinator is kept scrupulously clean, seeds are

washed and substrata (filter papers et al.) changed frequently there is normally

little necessity for special disinfectants. Old or slowly-germinating

seeds, species which are especially heavily infected with or sensitive to

moulds may be treated with one of the organic mercury dusts or sodium

hypo-chlorite before being placed to germinate. The use of disinfectants is

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further undesirable in that they often affect germination results by introducing

another variable. It is difficult to repeat tests under exactly identical

conditions. Boiling all glassware and germinator pads is the best precaution.

NURSERY

Nursery. This term is applied to a plot of land used for raising plants

that are intended for planting elsewhere for their land growth.

Soil and Cultivation. The best soil for a general nursery is a deep, rich

reasonably level, retentive upland. It is customary to grow most of the

nursery crops in rows, so that they may be readily cultivated. The land

should be ploughed deeply when the crop is planted and the surface soil

kept loose and fine during all the early part of the growing season, or until

about the middle of July. If the land that has to be used for a nursery is

rather shallow, it should be gradually deepened by ploughing from year to

year, and if inclined to dry out the addition of large quantities of organic

matter, together with constant cultivation, will do much to remedy these

defects.

The cultivation of a nursery or young forest plantation, provided the

latter is planted in rows, should be much the same as for garden crops and

consists in keeping the land stirred to the depth of three inches, thus giving

a dust blanket, which will protect from drought.

Grades of Nursery Stock

Nursery stock of different kinds has come to be known by such convenient

names as seedlings, transplants, street trees, forestpulled seedlings,

etc.

Seedlings are young plants, grown from seed, that have never been

transplanted, and are generally designated by their size or age. They form

the cheapest class of nursery stock, and are used largely for starting windbreaks.

Transplants are seedlings that have been at least once transplanted,

and are designated by their size and the number of times they have been

moved. They are higher in price than seedlings, but with some kinds of

trees they are much more likely to grow, and may be well worth the extra

price. Evergreens, especially Pines, will seldom do well unless once transplanted

before being set in a permanent place.

Street trees include the trees of large size which are used for street,

shade, and ornamental purposes. To be of the best quality they should have

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been transplanted two or more times and have received some attention in

the way of pruning so as to give them a good form. Such trees vary much

in quality and price, but the best are necessarily rather expensive.

Forest-pulled Deciduous Trees of small size can often be obtained at a

very low price, and may be as desirable as those that are nursery grown.

Forest-pulled Evergreen Seedlings of Small Size may also be desirable,

but too often they have poor roots, or have been so injured by poor

handling that they are worthless. They should have their roots carefully

protected at all times.

Forest-pulled Shade Trees sometimes grow very welt, but they are

always inferior to good nursery-grown trees. They are generally improved

by having their roots shortened two years before they are to be removed,

and when so treated grow very well.

TRANSPLANTING

Transplanting is Simply the Removal of the Plant. It may be to some

permanent place, as a park, lawn, or street, or it may be done for the purpose

of improving the root system and to give the tree more room to grow.

By shortening the long roots the root system is made more compact and

better able to withstand subsequent removal. This may be done by transplanting,

or by cutting around the tree with a spade or tree-digger. It is especially

desirable to do this to trees that are not easily moved on account to

their long branching roots, such as the Birch, or to those that have taproots,

like the Oak and Walnut. It is on account of their having had their

roots shortened so that the root system is compact and can all be moved

with the tree that nursery grown trees are generally superior to others.

In Transplanting it is important to take up a sufficient amount of roots

to support the plant, and as a rule the more roots the better the conditions

for growth. Very long roots should be shortened unless the tree is removed

to a permanent place, in which case all the good roots should be left on the

tree. All bruised or broken roots should be cut off in either case and the top

of the tree shortened to correspond. In transplanting trees, they should be

set at least one or two inches lower in the soil than they formerly stood,

and the roots should be spread out in the holes without crowding. It is customary

to plant many kinds of small trees in furrows made with a plough.

Very Large Trees (those over six inches in diameter) are sometimes

successfully planted in winter by taking them up with a ball of earth. This

is done by digging a trench around the tree, late in the autumn, deep

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enough to cut most of the roots, but far enough away from the tree to leave

a large ball of earth. The trench is then filled in with a mulch of some kind,

and when the ground is frozen the tree is moved, with the ball of earth attached,

to the hole which has been previously prepared and kept free from

frost.

After Trees Have Been Moved, or had their roots shortened in some

other way, they should generally not be transplanted again for at least one

or two years, during which time they will have overcome the injuries done

to their root system. The time which should thus elapse will vary with the

kind of tree, and also with the amount of injury done. Where the injury is

severe a much longer time will be required for recovery than where it is

slight.

Time to Transplant. Planting of trees should always be done when

they are dormant, or just as they start into growth in the spring, which is

generally from the middle to last of April. If for any reason it is desirable

to risk the moving of trees late in the spring, after the leaves started, they

should be cut back severely, all the leaves removed, and great pains be

taken to secure all the roots and to prevent their drying out. Very hardy deciduous

trees, as the Elm, Cottonwood, Boxelder and Ash, can often be

successfully moved in the fall if the ground is moist at the time of removal,

but great care must be taken to work the soil in very compactly between

the roots, so that there will be no large air-spaces among them. If the trees

are large, it is a good plan to stake them, so they cannot be blown about by

the wind.

Transplanting Evergreens. When seedling cone-bearing Evergreens

are two years old they should be transplanted and this should be done

about once in three years afterwards, until they are moved to their permanent

places. As Evergreens are very sensitive to being moved, this requires

more care with most deciduous trees. The most important point is to not

allow the roots to have even the appearance of being dry. In handling small

Evergreens in the field it is often a good plan to keep them in a pail containing

enough water to cover their roots or keep them in wet moss. They

may be transplanted in the spring, as soon as the ground works easily and

the roots have white tips, and they may be safely transplanted even up to

the time that the new growth shows three inches, but at this late date more

care must be taken in doing the work than when it is done earlier. Evergreens

can sometimes be moved successfully in August, or even in the autumn,

if they are to be carried only a short distance and the conditions of

the weather and land are favorable; but this is not a time for general planting,

and it is seldom advisable to do it at this season.

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Heeling-in. This term is applied to the temporary covering of the roots

of trees with earth to keep them from drying out after they are to be kept

only a few days, comparatively little care is needed in covering; but if they

are to be kept for several weeks or over winter, especially if the weather is

dry, great care must be taken to work the fine soil in among the roots and

to pack it solid. A good way of doing this is as follows: Select a dry, mellow

piece of ground and dig a trench just large enough to take in the roots

of the trees when laid close together in a single row. Place the trees or

seedlings in this trench in an upright position, a few at a time, and cover

the roots firmly and deeply with soil taken from close in front of the first

trench, thus making a trench for the next row. If trees that are of questionable

hardiness are to remain heeled-in all winter, it is a good plan to bend

the tops down and cover with earth. This is only necessary for winter protection.

The neglect to properly heel-in nursery stock as soon as it is received

is undoubtedly a frequent cause of failure. This is especially so in

the case of seedlings which are generally wintered in bundles, as they are

liable to dry out in winter. The bundles must be opened if large, and in any

case the soil should be packed in around them very solid by the feet or

otherwise.

FLOWER

A flower, also known as a bloom or blossom, is the reproductive

structure found in flowering plants. The biological function of a flower is

to mediate the union of male sperm with female ovum in order to produce

seeds. The process begins with pollination, is followed by fertilization,

leading to the formation and dispersal of the seeds.

In addition to serving as the reproductive organs of flowering plants,

flowers have long been admired and used by humans, mainly to beautify

their environment but also as a source of food.