128 REPRODUCTION IN PLANTS AND ANIMALS. 



doubtedly correspond to the unimpregnated ova of animals. 

 The line or ridge on the interior walls of the ovary to 

 which the ovules are attached, is called the placenta. The 

 ovule is attached to the placenta either directly or by 

 means of a prolongation or umbilical cord, termed the funi- 

 culus. The ovule when fully developed, consists of a coni- 

 cally shaped nucleus of cells containing a cavity, termed 

 the embryo sac, in its interior. This nucleus has two 

 coverings which have no organic connection with each 

 other excepting at tne base, where the vessels from each 

 covering meet, passing through the funiculus into the pla- 

 centa, which communicates with the nourishing walls of the 

 germen. This common point of union is called the chalaza. 

 At the apex of the ovule, both integuments leave an open- 

 ing which has been termed the foramen or micropyle. 



The reproductive function is exercised by animals and 

 plants when they have attained to the full development of 

 all their parts, or arrive at an adult state. The period 

 when this occurs varies greatly in each species and depends 

 entirely on the peculiarities of its constitution. When this 

 epoch arrives in plants, a visible change takes place in the 

 organic functions ; the stem ceases to elongate, and its 

 internodes no longer developing, the leaves remain crowded 

 together in closely approximated whorls, undergo peculiar 

 modifications in form and coloring, and a flower is pro- 

 duced. 



It is not the beauty and variety of the hues of flowers, 

 so much as the plan on wJiich they are constructed, which 

 is the chief point of attraction about them. The terminal 

 rosette of leaves called the flower, so different in size and 

 appearance from the ordinary leaves of the stem, is simply 

 metamorphosed stem leaves, crowded together in order that 

 they may act on each other. For example, all must have 



