18 MORPHOLOGY OF THE SEEDLING 



cotyledons as leaves in their nature, even if they are quite 

 unable to do the ordinary work of leaves. In seeds which have 

 endosperm, or food stored outside of the embryo, the cotyledons 

 usually become green and leaf-like, as they do, for example, in 

 the four-o'clock, the morning-glory, and the buckwheat ; but in 

 the seeds of the true grains, which contain endosperm, as in the 

 familiar instance of Indian corn, a large portion of the single 

 cotyledon remains throughout as a thickish mass buried in the 

 seed. In a few cases, as in the pea, there are scales instead of 

 true leaves formed on the first nodes above the cotyledons, and 

 co it is only at about the third node above that 



leaves of the ordinary kind appear. In the bean 

 and some other plants which in general bear one 

 leaf at a node along the stem, there is a pair 

 produced at the first node above the cotyledons, 

 and the leaves of this pair differ in shape from 

 those which arise from the succeeding portions 

 of the stem. 



25. Classification of plants by the number of 

 V their cotyledons. In the pine family the germi- 



FIG. 12. Germi- nating seed often displays more than two coty- 

 ledons, as shown in Fig. 12 ; in the majority of 

 common seed plants the seed contains two coty- 

 ledons, while in the lilies, the rushes, the sedges, the grasses, 

 and some other plants there is but one cotyledon. Upon these 

 facts is based the division of most seed plants into two great 

 groups : the dicotyledonous plants, which have two seed leaves, 

 and the monocotyledonous plants, which have one seed leaf. 

 Other important differences nearly always accompany the differ- 

 ence in number of cotyledons, as will be seen later. 



