If, 



His comparative table is as follows: — 

 Plant. 



Asexual Generation (lloweiing Sexnal Generation (fern 



plant or fern). prothallus). 



l.a;vo-albumins. Dextro-albumin. 



IJcxtro-starch. La^vo-starch. 



Dextro-sugars. La;vo-sugar.s. 



That there are also biological considerations is patent. 

 Why the life-energy of these sporogenous cells should at 

 this particular period in their history be diverted to the 

 formation of highly specialised reproductive cells, instead 

 of continuing to produce the simpler sporogenous ones, is 

 the more staggering question, of which the chemical aspect 

 would appear to be the complement. The fact of their 

 transformation is undoubted, and facts are stubborn 

 things. A later reference to this feature of alternation 

 will be made when considering the endosperm. 



THE MEGASPORANGIUM 



Is first observed as a primordial extension of the super- 

 ficial layer of dermatogen lining the ovary cavity, a little 

 to one side of the base. It is followed by hypodermal 

 periblem cells, and the nucellus thus differentiated (Fig. 

 12) is usually some 7-9 rows of cells across, including 

 the epidermis, each row being made up of six or more 

 cells. The median row soon bears evidence of archesporial 

 features in the activity and enlargement of its cells ; the 

 uppermost hypodermal cell broadens out at the top and 

 also elongates downwards, at the expense of surrounding 

 nucellus tissue, into a large wedge-shaped cell, the 

 archesporium or forerunner of the embryo-sac. In 

 numerous plants the archesporium develops directly into 

 the embryo-sac ; here, it divides twice, four cells being 

 derived from it, one of these, the lowest, becoming the 

 sac (Figs. 13-15). 



