320 



LYCOPODIALES 



sporangium (Fig. 165 B). The potential sporogenous tissue thus produced, 

 after successive sub-divisions, forms a very considerable sheet of tissue, 

 several cells in thickness. Of this, however, only a portion develops into 

 spores : in the case of a microsporangium certain tracts of cells of this 

 tissue assume dense protoplasm, and the cells, ultimately separating from one 



another, undergo the tetrad-division, producing 

 [microspores (Fig. 165 c, D); but other tracts of 

 cells, neither showing any regular outline or 

 arrangement, nor referable in origin to pre- 

 determined cells of the genetic tissue, become 

 less densely protoplasmic, and form the sterile 

 trabeculae : a tapetal tissue invests the fertile 

 tracts : it is derived partly from the innermost 

 layer of the sporangial wall, as in Lycopodium, 

 partly from the superficial cells of the trabeculae. 

 A similar differentiation of the potentially sporo- 

 genous -tissue is found also in the megasporangia, 

 the early stages of which are quite indistinguish- 

 able from those of the microsporangia ; but in 

 the former a relatively smaller number of cells, 

 usually lying isolated in the potential sporo- 

 genous tissue, and distributed with no constant 

 relation to their ultimate parent cells, enlarge 

 and divide to form the megaspores (Fig. 166). 

 As there is no opening mechanism in the 

 submerged sporangia of Isoetes, no basis for 

 comparison is yielded from that source. The 

 study of the development in Isoetes thus leads 

 clearly to the conclusion that there has been 

 a differentiation, within the sporangia, of tissues 

 at first of uniform character : that part of the 

 potential sporogenous tissue remains fertile, but 

 a large proportion in the microsporangium, and 

 a still larger proportion in the megasporangium, 

 is diverted to other uses, and remains sterile. 

 As regards the origin of the potential sporo- 

 genous tissue, and the form and position of the 

 sporangium, there is clear correspondence to the Lycopod-type, and 

 especially to those forms with the more bulky sporangia : in fact if we 

 imagine a heterosporous Lycopod, with its sporangium widened out radially 

 along the leaf-surface and its enlarged sporogenous tissue partly sterilised 

 so as to form trabeculae, the result would be practically what is seen in 

 Isoetes. 



A study of the sporangia of the fossil Lycopods is a necessary adjunct 

 to that of the modern forms, though the usual absence of developmental 



FIG. 166. 



Part of a section of a megaspor- 

 angium of Isoetes. The cell marked 

 (m) is the only fertile spore-mother- 

 cell, the rest are undergoing vegeta- 

 tive divisions, including the cell (a) 

 as shown by other sections of the 

 series. Thus sterilisation affects the 

 large majority of the cells of the 

 sporogenous group. X245. (After 

 Wilson Smith.) 



