294 



THIRD GROUP. — VASCULAR CRYPTOGAMS. 



trabeculae), and ihe fertile. But soon the cells of single rows lose their abundant 

 protoplasm and grow less rapidly, and their division results in the formation only of 

 elongated tabular cells. These are the trabeculae (Figs. 239, 240, 241, Tr). On the 



other hand the cells of the sporogenous rows 

 retain their abundance of protoplasm, and from 

 them are produced large masses of cells, the 

 mother-cells of the microspores. The trabeculae 

 meanwhile have become larger masses of tissue, 

 which are clearly distinguished from the sporo- 

 genous tissue by the small amount of protoplasm 

 in their cells, and by the intercellular spaces con- 

 taining air which lie between these. Here too 

 the sporogenous cells are surrounded by tapetal 

 cells which are in great part subsequently dis- 

 solved, as in the Ferns. The microspores are 

 formed by division of the mother-cell into four 



Fig 238. Selaginella inaequixli/olin. A nearly 

 mature macrosporangiiim, in wliich the fourth spore 

 lies behind and is not shown, raagn. looo times. 



parts. 



The macrosporangia follow the same course 

 of development as the microsporangia only 

 as far as the formation of the archesporium, and the trabeculae are formed in 

 the same way in both. The fertile cells of the archesporium form by transverse 

 division on the side towards the wall of the sporangium a few cells which remain 

 sterile; each fertile archesporial cell produces only one sporogenous cell, which 

 by the process just described is sunk in the tissue of the sporangium. Hence the 

 mother-cells of the macrospores are isolated, and they each produce four macrospores. 

 The mother-cell of the macrospores is distinguished from all the other cells by its 



FIG. 239. Isoetes lacttstris, development of microsporangia. A portion of a Wngitudinal section with the cells of 

 archesporium shaded ; the vascular bundle of the sporophyll would adjoin on the left. B and C portions of transverse 

 sections, in which the groups of sporogenous cells formed from the archesporium are likewise shaded dark; C tapetal 

 cells, TV trabeculae. 



superior size and by the protoplasm which it contains. It is at first polygonal, but 

 afterwards becomes round and then begins to exercise a destructive influence on 

 the neighbouring cells, especially the tapetal cells. These cells separate from one 

 another, assume a spherical form and are ultimately dissolved, so that the mother-cell 

 comes to lie in a cavity, and there divides into four daughter-cells, the macrospores. 



