26 HEREDITY AND EVOLUTION IN PLANTS 



18. Development of the Fertilized Egg.^ — After fertili- 

 zation the egg begins to develop, undergoing a series of 

 nuclear and cell-divisions, accompanied by increase in 

 size. The cell-wall of the first division (in all of the family 

 Pol)^odiaceae) is parallel to the axis of the archegonial 

 neck. The second wall, at right angles to the first, di- 

 vides the oosperm into four cells. The beginning of these 

 divisions marks the beginning of the embryo. By further 

 cell-divisions each of the first four cells develops a mass of 

 embryonic tissue. The two cells on one side of the first 

 wall formed represent, the one the embryonic stem, and 

 the other the embryonic leaf, or cotyledon. One of the 

 two cells on the opposite side of the first wall, develops 

 into the embryonic root, while the other develops into an 

 organ peculiar to the embryonic stage, and known as the 

 foot (Fig. 25). The function of the foot is to absorb 

 nourishment for the young embryo from the prothallus. 

 The need of such an organ becomes apparent when it is 

 recalled that the oosperm, and consequently the embryo, 

 lie free in the venter of the archegonium, without any 

 organic or structural connection with the prothallus. 

 This necessary connection is early established by the foot. 



19. Growth of the Embryo. — ^As the embryo continues 

 to grow, the root develops first. The advantage of this 

 wiU become evident when we remember that the primary 

 and most fundamental need of the young plant is water, 

 which is taken in by the roots; The next most funda- 

 mental need is nourishment, and as plant food is manufac- 

 tured in chlorophyll-bearing organs, and usually in 

 leaves, we would expect the early development of leaves. 

 Such is the case, the growth of the first leaf being second- 

 ary only to that of the root, and in advance of the stem. 



