438 PART III. THE CLASSIFICATION OF PLANTS. 



of the macrosporangium or sometimes two such sterile cells 

 which, with or without further division, represent a tapetal layer. 

 The large remaining cell now undergoes division 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 some plants (e.g. Senecio and other 

 gamopetalous Dicotyledons; Orchis pall ens, Gymnadenia Conopsea, 

 and some Grasses, among Monocotyledons) no tapetal cells are 

 cut off. In a few plants (Cycads, Callitris quadrivalvis, Cupreous 

 sempervirens, among Gymnosperms ; Casuarina and some other 

 Amentales, among Angiosperms) 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. 



Generally 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. In some 

 exceptional cas3S (e.g. Rosa livida, Casuarina, Taxus, Ginkgo, 

 Thuja, Gnetum) several of the potential mother-cells begin, at 

 least, to develope, though even in these cases only one of them, 

 as a rule, ultimately gives rise to a fertile macrospore. 



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. But Casuarina is an exception 

 in this respect ; here the numerous (as many as twenty) mother- 

 cells all grow with equal vigour, and, as they do not attain a 

 large size, there is space for them all in the macrosporangium. 

 Casuarina is further peculiar in that some of the small sterile 

 cells of the sporogenous tissue develope into tracheidal cells 

 somewhat resembling the elaters of the Hepaticee. 



The macrosporangium is indehiscent, and only becomes de- 

 tached from the plant after it has developed into a seed. 



The macrospore or embryo-sac is, as a rule, developed singly in 

 the macrosporangium ; and, further, it is always developed 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 



