THE EVOLUTION OF PLANT REPRODUCTION 



75 



defined parts. There is a rudimentary foliage shoot known as the 

 PLUMULE, an equally rudimentary root known as the radicle, 

 and two seed-leaves or cotyledons (fig. 49). Those Angiosperms 

 in which the embryo develops two cotyledons, as in the apple, are 

 known as Dicots; in other Angiosperms the embryo produces but 

 one cotyledon, this group being known therefore as the Monocots. 

 Lilies, orchids, palms, grasses are Monocots; while oaks, roses, 

 buttercups, daisies are representative Dicots. 



Other changes in the flower include a withering away of 

 the top of the pistil. But the most vital changes take place in the 

 ovary of the pistil. Here the embryo sporophyte is rapidly in- 



COTYLEDON 



BEAN (opened) corn (section) 



Fig. 49. — Some seeds contain embryos with two cotyledons, as occurs in the 

 Dicot seed plants (bean); others contain embryos with a single cotyledon, as 

 in the Monocot group (corn). 



creasing in size, therefore requiring a considerable supply of food 

 which must come from adjacent tissues in the ovule. While the 

 embryo was developing from the fertilized egg, the female pro- 

 thallus cells became a storehouse for reserve food, .known as the 

 ENDOSPERM. The growing embryo now begins to penetrate this 

 endosperm region of the ovule, absorbing food from it. In most 

 of the Monocots the embryo ceases absorption and growth before 

 the endosperm is entirely consumed, so that when the embryo 

 becomes dormant it is surrounded by the remaining endosperm. 

 In the Dicots, the usual condition is that the embryo advances 

 into the endosperm until it is all used up; some of this absorbed 

 food is stored in the cotyledons of the embryo which thus become 

 large fleshy structures. The integument of the ovule forms a 

 protective covering to the embryo, both together forming one 



