34 



ELEMENTARY BIOLOGY 



The function of the cotyledons seems to be confined in most 

 cases to the holding of reserve food, which is drawn upon 

 by the baby plant until it is developed far enough to get food 

 for itself. 



In some kinds of seeds the cotyledons are very thin ; in 

 such cases we usually find that there is a mass of food material 

 packed in all around the embryo. A mass of food thus placed 

 about the embryo is called the endosperm, which means 

 " within the seed." The grains and the castor seed are good 

 examples of seeds that contain endosperm (Fig. 7). 



When we compare the embryo of a 

 grain, such as the corn, with the other 

 embryos that have been mentioned, 

 we find one great difference in the 

 structure. The grain has but a single 

 cotyledon. This is rather large, though 

 not fleshy, and only the tip comes out 

 of the seed covering as the first leaf. 

 The base remains in contact with the 

 endosperm and serves as an " absorb- 

 ing organ," withdrawing food material 

 from endosperm and transferring it to 

 the growing plant. 



There are many plants, besides the 

 grains, that have but one cotyledon in 

 the seed. This fact would not seem to be of any great importance 

 by itself, but it is connected with so many other characters, such 

 as the veins in the leaves, the structure of the stem, the structure of 

 the flower, and general habits of life, that we sometimes designate 

 one of the main divisions of seed-bearing plants as the monocotyls, 

 meaning the " one-cotyls," and another as the dicotyls, or " two- 

 cotyledon " plants. Among the dicotyls are included most common 

 weeds and cultivated plants, outside of grains. 



The seeds of the plants belonging to the pine family (fir, spruce, 

 hemlock, etc.) have usually several cotyledons, and this family is 

 accordingly designated as the polycotyls in some books, this name 

 meaning " many cotyledons " (see Fig. 6). 



FIG. 7. Seeds with endosperms 



/, asparagus; 2, poppy; j, pine; 



4, maize, or Indian corn. (All shown 



in longitudinal section) 



