xiv ISOETACEZE 559 



rogenous cells. As in Lycopodiuni and Selaginclla, the tapetal 

 cells remain intact, instead of being broken down as they usually 

 are in the Ferns and Equisetum. 



In the microsr/orangium all the sporogenous cells divide, 

 the divisions being successive and usually resulting in spores, of 

 the "bilateral" type, although tetrahedral spores are sometimes 

 formed. The number of spores in each sporangium is very 

 great. In /. echinospora, it ranges from 150,000 to 300,000. 



The Macro sporangium 



The earliest stages of both types of sporangium are alike, 

 but the macrosporangia are recognisable as such earlier than 

 the microsporangia. In the former, before any distinction of 

 fertile and sterile tissue is evident, certain cells become notice- 

 ably larger than their neighbours, and enter into competition, as 

 it were, to become the spore mother cells. There is apparently 

 no rule as to either the number or position of these potential 

 mother cells; but sooner or later some of them outstrip their 

 competitors, become very large, and ultimately divide into the 

 four macrospores. 



The formation of the trabeculse and tapetum is essentially 

 the same as in the microsporangium ; but the trabeculse are fewer 

 and more massive, and the tapetum is several cells in thickness. 

 The unsuccessful sporogenous cells probably are used up in the 

 further development of the growing spores. 



The further development of the macrospore has been studied 

 in I. Durieui by Fitting ( i ) . Preliminary to the first nuclear 

 division in the mother cell, whose membrane consists of a pec- 

 tose-compound and not cellulose, there is a division of the starch 

 granules into two groups which divide again, and the four 

 starch masses arrange themselves tetrad-wise in a way that 

 recalls the behaviour of the cell contents in the dividing spore 

 mother cells oi.Anthoceros. The four nuclei resulting from the 

 repeated division of the primary nucleus are in close contact 

 with the four starch masses, and there then follows the simul- 

 taneous formation of cell plates between the nuclei. The cell 

 plates are replaced by the cell walls which separate the four 

 young tetrahedral macrospores. 



The protoplast of each young spore secretes about itself a 

 special membrane from which is later developed the characteris- 



