ANGIOSPERMS 



347 



The seed. In the ripened seed of Capsella the embryo fills the 

 entire cavity of the embryo sac ; the food reserve necessary for 

 its growth during seed germination is stored in the cotyledons, 

 as in the pea and bean. The embryo, as in the latter seeds, has 

 two cotyledons, a plumule (or first terminal bud), a hypocotyl, 

 and the root meristem at the tip of the hypocotyl. The seed 

 coats are formed of cells with greatly thickened walls, which 



ale cell fc- 

 ,( sperm) 



Tube nucleus 

 Pollen tube''' 



FIG. 204. Ovule, pollen tube, and fertilization in Iris and a lily 



a, ovule of Iris with embryo sac and female gametopliyte at the time of fertiliza- 

 tion ; 6, double fertilization in a lily ; c, pollen tube of Iris, b, after Gu'inguard ; 

 a and c, from original drawings by M. Louise Sawyer 



effectually protect the embryo during its period of rest. The 

 germination of the seed and the adjustments of the seedling to 

 the environment are essentially the same as in the bean. 



Life history. It is evident from the above discussion that the 

 angiosperms have the same general stages in their life history as 

 the spruce. The new features relate to details of structure and 

 reproduction already discussed and hence need only a brief 

 treatment in the form of a summary at this point. 



The flower of the angiosperms is a Highly modified strobilus in 

 which the microsporophylls have been transformed into stamens 

 and the megasporophylls into one or many pistils. The closed 

 pistil, with the stigma differentiated for the reception of pollen, 

 is the most distinctive feature of the angiosperms, although the 

 perianth, when developed, is an important characteristic. The 



