true of most plant and animal foods that are eaten in a fresh state.

at first they seemed to serve as leaves, later becoming absorbed as food by the growing seed. In the pea the cotyledons serve as food, but remain under ground. In the corn the single cotyledon serves as an organ for digesting and absorbing food from the storehouse of food known as the endosperm.

Monocotyledons and Dicotyledons. Plants that bear seeds having but a single cotyledon are called monocotyledons. The corn is an example of such a plant. Although we find a good many monocotyledonous plants in this part of the world, this group is characteristic of the tropics, just as the dicotyledons are the type for the temperate climate. Sugar cane and many of the large trees, such as the date palm, palmetto, and banana, are examples. Among the common monocotyledons of the north temperate zone are corn, lily, hothouse smilax, and asparagus.

Polycotyledons. A third type of plant, grouped according to the number of cotyledons, is the group represented by the pines and their kin.

Pine seedlings may be grown in damp moss or sawdust. They must be started at least three weeks before they are needed for use in the laboratory.' The Pine Cone and its Seeds. - Ma

terial should be gathered in the fall and early summer. Get some very young cones and some of older growth that contain seeds. The pine tree bears inconspicuous flowers of two kinds, pollen-bearing and seed-forming. Pollination is accomplished by the wind, the cones growing as the result of fertilization. Notice the position of the cone on the branch. Compare a young cone with an old one. You will find in the young cones that the scales are green in color and are cemented together by the sticky resin or pitch. In the older cones the seeds are ready for dispersal. They usually take two summers to grow to maturity. Pull back one of the scales making up the cone and see what happens. How is the seed adapted to be scattered? Draw one of the scales of the cone and a winged seed to show the position of the seed when in place. If you cut open a seed lengthwise, after having split the hard outer coat, you will find the tiny embryo in the center of the seed, surrounded by its

1 See Hunter and Valentine, Manual, page 75.

endosperm. If the student is a very careful observer, he may be able to make out the number of cotyledons in the young plant. There are fifteen seed leaves in one common species of pine. The number and position are better seen in a young seedling of three or four weeks' growth.

The Uses of Seeds. Some of the uses of seeds to man have already been noted. A seed is a very young plant usually provided with a store of food to give it a start in life. Its use to the parent plant is incalculable, for it is by means of the seed that a plant reproduces its kind. This can be done, as we shall see later, to a limited degree by cuttings, grafting, and in other ways, but the usual way is by the production and planting of seeds. Not only does a seed serve to continue a species of plant in a certain locality, but it serves to give the plant a foothold in new places. Seeds may be blown. by the wind or carried by animals, or by a hundred devices work their way to pastures new, there to establish outposts of their kind.

Pine seedling.

Immense numbers of seeds may be produced by a single plant. This may be of great economic importance. A single pea plant may produce twenty pods, each containing from six to eight seeds. This would mean the possibility of nearly twenty-five thousand plants produced from the original parent by the end of the second season. A plant of Indian corn may produce over fifteen hundred grains of corn. On the other hand, many weeds produce seed in still greater numbers. A single milkweed may set free over two thousand seeds. A single capsule of Jimson weed has been found to hold over six hundred seeds. The thistle is even more prolific.

Some seeds, especially those of weeds, are able to withstand great extremes of heat and cold and still to retain their ability to germinate. Some have been known to retain their vitality for over fifty years. In plants, the seeds of which show unusual hardiness, it is found that the food supply is often so placed as

to protect the delicate parts of the embryo from injury. The food is in a form not easily dissolved by water or broken up by the action of frost, so that it is kept in a hard state until such a time as it can be softened by the process of digestion during the growth of the plant. It can be seen that plants bearing seeds having some of the above characters have a great advantage over plants bearing seeds that are poorly protected.

We

External Factors which determine the Growth of Seeds.1 have spent some time in the consideration of seeds simply to learn a little about their structure. This has been done so that we may understand the work as we take up, by means of the following experiments, some of the factors which call the dormant seed to life. We know that a dry seed, after lying dormant and apparently dead for months and sometimes for years, will, when

1 In making experiments it is important to exclude, if possible, all other factors but the one you wish to determine.

the proper stimuli are applied to it, start in its growth into a new plant. Let us see what these stimuli are.

Effect of Water on Dry Seeds. Weigh ten dry navy beans; leave them in water over night; reweigh. How much have they increased in weight? We have already found that water gets into the seed through the micropyle. It can be proved that it gets in through the seed coat as well. If you cover five seeds entirely with paraffin, and in five others cover the hilum, micropyle, and half of the remainder of the coat, a difference in weight and size will be apparent the next morning.

Expansive Force of Germinating Seeds.-The expansive force of germinating seeds is considerable. You have noticed that the bean is considerably larger after soaking.

Fill a small bottle almost full of dry seeds (beans or peas), then fill the space left with water; wire in the cork tightly. Leave the bottle overnight and note the results next morning. Would this force be of use in getting a start under the soil? Have you noticed that the soil is lifted in the garden by the rows of germinating peas and beans just before they come up? Is this due entirely to the expansive force of seeds? Watch future experiments before you attempt a definite an

The expansive force of germinating seeds. The flower pot to the left was filled with dry beans, a block of wood wired on, and the whole apparatus placed in a pail of water over night. The right-hand figure shows the result.

swer.

Will a Dry Seed germinate?Place a layer of moist blotting paper or sawdust in bottom of each of three cups or three tin cans. Soak fifteen navy beans overnight. Place five in each dish. Water one dish so as to cover the seeds; water the second SO as to keep sawdust rather moist; let the third remain unwatered. Cover the cups with loose-fitting covers. Make daily observations of the number germinating, and the condition of each for at least ten days. Put the results in tabular form. What amount of water is most favorable for germination of the navy bean?1

Water a Factor in Germination. A dry seed will not germinate. Water is absolutely necessary to start the forces at work toward growth. But it is sometimes difficult to determine the amount of water that is most favorable to germination. Some seeds require a great deal of water, others require very little.

Will a Seed grow without Air?- We have already seen that in the germinating corn plant the starch stored in the endosperm was changed to grape sugar by the action of a digestive ferment called diastase. This sugar was then used by the plant as food. 1 See Hunter and Valentine, Manual, page 222.