PHANEROOAMTA. 



253 



nourished by the starch in the cotyledons, which in this 

 species remain during the whole process of germination 

 beneath the ground enclosed in the seed-coat. In the com- 

 mon Field-bean (Phaseolus) the germination is the same 

 excepting that the stem elongates below the cotyledons 

 and brings the latter above the ground. 



531. The seed of the Castor-oil Plant contains a large 

 embryo surrounded by a thin 

 layer of endosperm (Fig. 141, 

 I). In its germination the 

 root and stem below the coty- 

 ledons elongate, and thus bring 

 the seed-coat with the con- 

 tained cotyledons above the 

 ground (Fig. 141, II). The 

 cotyledons remain within the 

 seed-coat until they have ab- 

 sorbed all of the endosperm; 

 when this is accomplished the 



1 . J- ^^ J Fio. 142— Magnified fragment of 



empty seed-coat tails away, and a leaf of a Dicotyledon, showing re- 

 _ _ , tlculated venation. 



the freed cotyledons expand 



and assume to some extent the function of ordinary foliage- 

 leaves. 



632. The venation of the leaves of Dicotyledons is easily 

 studied by macerating them so as to remove the soft tissue, 

 leaving only the fibro-vascular bundles. While there is as 

 a rule a general likeness between them, there is yet an 

 almost infinite diversity in the details of structure. The 

 general disposition of the smaller veins is well illustratedl 

 by Fig. 142. 



533. There are now known upwards of 78,000 Dicotylei 

 dons, showing every degree of deyelopment from minute 



