GROUP III. PTERIDOPHYTA : FILICIN^E ; EUSPORANGIATJ3. 387 



lutely exogenous, is superficial. In this respect Isoetes would differ from the 

 other Filicinse. 



The first indication of the development of the members is the segmentation 

 of each octant after the manner of a tetrahedral apical cell, so that for a short 

 period, stem, root, and leaf may be said to grow by means of an apical cell. 

 By the formation of periclinal walls, these cells are soon converted into a 

 merismatic tissue. 



The growth of the cotyledon and of the first root is rapid ; that of the stem 

 slow : the cotyledon and the first root soon make their way out of the macro- 

 spore, when the former becomes green and the latter curves into the soil : the 

 growing-point of the stem is deeply seated between the bases of the cotyledon 

 and of the root. A second leaf is early developed from the growing-point of 

 the stem and is opposite to the cotyledon : these two first leaves are smaller than 

 those produced later, and have only two longitudinal rows of air-chambers 

 instead of four. The second root is developed endogenously from a group of 

 cells at the base of the second leaf : it is clearly adventitious. 



GA.METOPHYTE. As Isoetes is heterosporous, the gametophyte is represented 

 by distinct male and female individuals, which remain connected with the spores 

 producing them. 



The male individual is developed from a microspore. The microspore which 

 has the form of the quadrant of a sphere and is consequently of the bilateral or 

 radial type undergoes, on germination, division by a transverse wall, formed 

 near one of its somewhat pointed ends, into two cells, a large and a small : the 

 latter is the vegetative cell, and undergoes no further change ; the former is 

 the mother-cell of the male organ or antheridium. The prothallium here is thus 

 very much reduced, consisting of a single antheridium and of a single purely 

 vegetative cell. The antheridium, developed by the growth and division of the 

 mother-cell, consists of four peripheral cells forming the wall, and of four 

 central cells, each of which gives rise to a single spirally coiled multiciliate 

 spermatozoid. 



The female individual is developed from a maerospore. The macrospores are 

 much larger than the microspores, and are nearly globular in form, though 

 they belong to the tetrahedral type, as can be seen by the three ridges on 

 the spore where it was in contact with the other three developed from the 

 same mother-cell. On germination, the nucleus of the maerospore undergoes 

 repeated division ; this is followed by free cell- formation in the apical region 

 (the pointed end where the three ridges meet) of the maerospore, the result 

 being the formation of a small-celled tissue ; subsequently cell-formation 

 extends into the basal portion of the spore, a tissue being formed there con- 

 sisting of relatively large cells with coarsely granular contents. Thus the 

 maerospore becomes completely filled with a mass of cellular tissue which con- 

 stitutes the female prothallium : the upper small-celled tissue is the essentially 

 reproductive portion, whilst the lower large-celled tissue simply serves as a 

 depository of nutritive substances. 



The female organ, the archegonium, is developed from one of the 

 superficial cells of the small-celled prothallial tissue, after the manner de- 

 scribed on page 377. It appears that two or three archegonia are usually 

 formed : but if none of these primary archegonia are fertilised, a small number 

 of additional archegonia may be subsequently developed. 



