328 



GENERAL PRINCIPLES 



paradox disappears when we recognize that the " cavities" 

 mentioned are not so much spaces as cleavage planes between 

 organs, which permit the involution (growing inward) of other 

 organs. 



708. As examples of the first type we may take a fern and a 

 dicotyledon. The fertilized egg cell of the fern divides into 

 two cells, and these divide again, making four. These four 

 cells continue to divide indefinitely, and the cells remain a 



FIG. 188. 



FIG. 189. 



FIG. 188. Development of the fern embryo. A, The egg cell divided into 

 quadrants; B, a later stage, the four quadrants still evident and from them 

 develop the four parts of the plant as indicated. /, Develops the foot; r, develops 

 the root; s, develops the stem; /, develops the first leaf. 



FIG. 189. Development of the dicotyledon embryo, i and 2, Early stages 

 showing all of the suspensor; 3 and 4, only the end of the suspensor is shown. 

 In 2 the embryo is marked off at the upper end of the suspensor. In 3 the em- 

 bryo is farther advanced. In 4 the root (T), stem (5) and cotyledons (C) are 

 distinguishable. 



single solid mass. From each quadrant of the four-cell stage 

 a definite part of the young plant develops; that is, from I 

 develops the root, from II the stem, from III the first leaf, and 

 from IV the foot, an organ by which the plantlet continues to 

 draw nourishment from the mother prothallus. This foot is a 

 type of structure which is very common among plants and ani- 

 mals; that is, of structures which are functional only in the 



