354 BOTANICAL GAZETTE [November 



especially at the angles formed by the mutual pressure of the 

 sporangial lobes. 



As the sporogenous tissue increases in size there is pressure upon 

 the wall cells, and they become flattened and extended, so that at 

 the time of their maximum development they are broad thin plates. 

 By the time of spore formation they are usually quite flattened (fig. 

 36), and during the further growth of the spores become more or 

 less disorganized, so that by the time the spores have reached 

 maturity, just before shedding, the wall has become a very thin 

 layer abutting the epidermis, which has now become, in effect, the 

 functional sporangium wall. 



Tapetum. — At the time when the sporangium wall is completed 

 the sporogenous tissue inclosed within it is uniform in appearance 

 (fig. 34). Soon, however, the peripheral layer of this tissue becomes 

 differentiated as a tapetum (fig. 35), thus originating from the 

 sporogenous tissue and not from the inner layer of the sporangium 

 wall, as in some forms. The tapetum, however, has its chief 

 significance from a physiological standpoint, being generally 

 regarded as a nutritive layer, its origin seeming to be of little 

 morphological significance. The tapetal cells are usually uni- 

 nucleate, but not infrequently are binucleate (figs. 35-36). The 

 tapetum is quite distinct during later phases of the development of 

 the sporogenous tissue, is sharply differentiated at the spore mother 

 cell stage, as pointed out by Chamberlain (3), and remains distinct 

 during the early winter (fig. 36). With the growth of the 

 microspores in the spring it becomes less and less prominent, until 

 near pollination it consists of only a very thin layer of disorganized 

 material surrounding the spore mass. 



EPIDERMIS 



From the beginning of the primordium to the mature sporophyll 

 the epidermis remains as a distinct layer, the sporangium develop- 

 ing from hypodermal tissues, as already stated. During the early 

 growth of the sporophyll the epidermis is meristematic throughout, 

 dividing anticlinally (fig. 24), its surface area thus keeping pace with 

 the increase in the mass of the sporogenous tissue. An occasional 

 periclinal division results in the epidermis becoming 2 cells thick at 



