LIVING CONDITIONS; THE SEED 



69 



60. Food in seeds. The concentrated food found in seeds of 

 common plants is of interest to us in three ways : First of all, 

 we may infer that this food is actually used by the young plant 

 until it is able to provide for itself. That this is a sound infer- 

 ence may be tested by separating from several seedlings the 

 "food reserve." Next we can observe that the cotyledons in 

 such plants as the beans and peas 

 do actually shrivel away as the 

 plant becomes larger. The con- 

 tents of the corn grain also dis- 

 appear as the seedling develops. 

 Finally, by means of chemical 

 experiments we can see that the 

 changes taking place in the food 

 masses of the seedlings are of the 

 kind that we should expect to find 

 if the food were actually being 

 transported to the growing portions 

 (see section 105). 



61. Seedlings. Examine a few 

 seeds that have been planted two 

 or three days, and you will see that 

 the hypocotyl has emerged and is 

 assuming the appearance of a root. 

 At the other end of the embryo you 

 may see the unfolding epicotyl. If 

 we examine different stages of peas, 

 squash, oats, corn, beans, and so on, 

 we shall be able to see a great variety of methods by which the 

 young plant crawls out of its covering and establishes itself in 

 the soil (Fig. 35). 



Large seeds, containing a large amount of reserve food, are 

 apparently at an advantage, since they may develop more root 

 and more shoot before they are overtaken by the necessity of 

 providing themselves with food. We should therefore expect 

 that plants with large seeds would be, on the whole, more 



Fig. 34. Seeds with 

 endosperms 



/, asparagus; 2, poppy: 3, pine; 

 4, maize, or Indian corn. In some 

 kinds of seeds the cotyledons are 

 very thin. In such cases we usu- 

 ally find that there is a great deal 

 of food material surrounding the 

 embryo, whereas in seeds with 

 fleshy cotyledons there is food 

 packed within the cotyledon. The 

 food packed around the seed is 

 called endosperm, which means 

 "within the seed." (All shown in 

 longitudinal section) 



