XIV ISOETACEJE 543 



Because of the greater number of free nuclei in the upper part 

 of the spore, and their consequent close proximity, the cells 

 are smaller than those in the central and basal parts of the pro- 

 thallium. Sometimes the transition from this small-celled tissue 

 to the large-celled tissue of the basal part is quite abrupt and 

 the more noticeable as the upper cells are more transparent ; but 

 there was nothing to indicate that this was in any way con- 

 nected with the early divisions of the primary nucleus, and more 

 often no such sudden transition was seen. 



Hofmeister's account of the coalescence of previously sepa- 

 rate cells to form the prothallium was obviously based upon 

 incorrect observation, and is not borne out by a study of sections 

 of the germinating spore. 



The first archegonium is very early evident, generally be- 

 fore the cell division is complete in the lower part of the spore. 

 It occupies the apex of the prothallium, and the mother cell is 

 distinguished by its large size and dense granular contents. 

 It is simply one of the first-formed cells that soon ceases to 

 divide, and as its neighbours divide rapidly the contrast between 

 them becomes very marked. Whether seen from above or in 

 longitudinal section, it generally is triangular, or nearly so. In 

 the structure of the mature archegonium, Ophioglossum shows 

 strong points of resemblance, -as do the Marattiacese, but the 

 egg cell is much larger in Isoetes. 



The development of the archegonium corresponds almost 

 exactly with that of Marattia, but the basal cell is always want- 

 ing, and the first transverse wall separates the central cell from 

 the cover cell. The first division in the inner cell is parallel 

 with the base of the cover cell, and divides it into the primary 

 canal cell and central cell. The contents of the three cells of 

 which the archegonium is now composed are similar, and the 

 nuclei large and distinct. The cover cell next divides into four 

 by transverse walls (Fig. 311, E), and from these, as in Marat- 

 tia, the four rows of cells of the neck are formed. The number 

 in each row is usually four in the mature archegonium. The 

 ventral canal cell, which like that of Maraftia extends the whole 

 breadth of the central cell, is separated almost simultaneously 

 with the appearance of the first transverse divisions in the neck 

 cells. The neck canal cell has at first a single nucleus, which 

 later divides, but there is no division wall formed. Although 

 the number of cells in each row of the neck is usually greater 



