400 PART IV. CLASSIFICATION. 



into two by a transverse wall, and one or both of these cells may 

 divide in a similar manner. Thus a longitudinal row of large 

 cells, two to four in number, is formed, all of which are potentially 

 mother-cells of macrospores. In a few plants (Cycads and some 

 Couiferae among Gymuosperms ; some Amentales, among Angio- 

 sperms) the growth of the archesporial cells is more extensive, 

 leading to the production of a considerable mass of sporogenous 

 tissue, as in the macrosporangia of the Pteridophyta. 



Generall} r speaking, only one of the cells of the sporogenous 

 tissue shows any sign of developing into a macrospore ; and in the 

 normal Angiosperms, this cell is generally the lowest (nearest the 

 chalaza) of the longitudinal row described above. The growth of 

 the fertile mother-cell of the macrospore is vigorous. It causes the 

 displacement and absorption of the sterile cells of the sporogenous 

 tissue. 



The macrosporangium is indehiscent, and only becomes detached 

 from the plant after it has developed into a seed. 



The macrospore (megaspore) or embryo-sac is, as a rule, deve- 

 loped singly in the macrosporangium ; and, further, it is always de- 

 veloped singly from its mother-cell without any indication of that 

 division into four, which is characteristic of the development of 

 spores in general. It is in fact impossible, as a rule, to fix upon 

 any stage at which the transition from macrospore-mother-cell to 

 macrospore may be considered to take place ; for the mother-cell 

 simply grows and becomes the macrospore without any special 

 differentiation. However, in the Cycadaceae, the wall of the 

 mother-cell undergoes that differentiation which is characteristic 

 of spores, so that the wall of the macrospore consists of two layers 

 the outer of which is cuticularised. The macrospore is simply a 

 large cell, containing vacuolated protoplasm in which lies a 

 nucleus, and having, as a rule, a wall of cellulose. 



In the course of its growth, the macrospore frequently causes the 

 absorption of more or less of the tissue of the nucellus, more 

 especially towards the micropylar end. It commonly attains 

 such a size that little or none of the nucellar tissue remains : 

 in some cases, however (e.g. Gymnosperms, Scitaminese, most 

 Xymphseacese), the macrospore does not grow to such an extent, so 

 that a considerable mass of nucellar tissue is left, which persists to 

 some extent in the seed as perisperm, its cells being then filled 

 with nutritive substances. 



General Histology. The following are the principal characteris- 



