14 



of their nuclei is about coincident with the megaspore 

 tetrad, and the spireme stage may be found as late as the 

 first division of the embryo-sac mother-cell. However, if 

 may be stated generally that the mitoses of the megaspore 

 and microspore nuclei run nearly concurrently, so that 

 when the embryo-sac is mature the unshed pollen grains 

 are complete, and contain the tube nucleus and the two 

 generative or male nuclei (Figs. 9-1 la). It is thus observed 

 that the early microspore mother-cell stages are prolonged 

 during the passage of the archesporial cell of the niega- 

 sporangium through its tetrad-forming divisions up to the 

 differentiation of the functioning megaspore or embryo-sac 

 cell. 



Let us for a moment revert to the significance of the 

 mysterious transition of the sporogenous cells directly 

 into spore mother-cells without apparent changes observ- 

 able microscopically. The cells they now produce become 

 altogether different in character and function from 

 sporogenous cells, phenomena comparaDle with the equally 

 remarkable changes that take place in the development of 

 the sporophyte embryo from the fertilised ^^^. Just what 

 happens in them, and when the change actually occurs, are, 

 as yet, beyond our ken. These problems, closely 

 associated with the phenomenon of alternation of genera- 

 tions, are largely physiological ; and Dr. John Beard, in 

 his most recent pamphlet " On the Asymmetry of Life,"- 

 has some cogent suggestions as to the part the asymmetric 

 carbon atom may play in the stereo-chemical aspects of 

 the constitution of their saccharine, amyloid and 

 albuminous food substances. There is \\ork here for the 

 bio-chemist to confirm or disprove the contention that the 

 dextro-sugars, dextro-starch and leevo-albumins of the 

 sporophyte are replaced by their opposites, laevo-sugars, 

 laevo-starch and dextro-albumin, in the gametophyte. 



