558 MOSSES AND FERNS chap. 



arrangement of the cells described by Goebel. He says (1. c, 

 p. 241), ''I am forced to conclude that the sporangium-of Isoetes 

 (at least of /. echinospora and /. Engelmanni) just as the 

 microsporangium of Angiosperms, grows as a unit, and not as 

 a number of individual segments." 



The velum appears very early and is apparently developed 

 directly from a part of the sporangium-fundament — indeed it 

 looks as if in some cases it actually contributed to the sporoge- 

 nous tissue. The velum reaches its full development before the 

 rest of the sporangium does. In certain species, some of its 

 cells, as well as those of the adjacent leaf-tissues, may become 

 lignified and show spiral and annular thickenings. 



In their early stages, there is no difference between micro- 

 and macrosporangia. Wilson-Smith could find no indication 

 in the species investigated by him, of the early differentiation 

 of the two kinds of sporangia described by the early investi- 

 gators. In both macro- and microsporangia, divisions occur 

 in all directions, resulting in a very large mass of potential spo- 

 rogenous tissue. There is later, however, a differentiation of 

 the archesporial tissue into fertile and sterile areas, the latter 

 forming later the "trabeculse." 



About the time that the last cell-divisions are taking place In 

 the archesporial tissue, certain regions divide less actively and 

 react less strongly to stains. These relatively inactive regions 

 are the sterile ones, and from them are developed the sporan- 

 gium wall, the trabeculse and tapetum, while the. rest of the 

 archesporial tissue, at least in the microsporangium, develops 

 spores. 



The trabeculse are more or less irregular masses of tissue, 

 not forming definite partitions, although they may anastomose 

 more or less freely (Fig. 322, C). The cells of the trabecula 

 become flattened and extended by the subsequent growth of the 

 sporangium, and lose to a great extent their protoplasmic con- 

 tents, so that they soon become clearly separated from the inter- 

 vening sporogenous cells. The trabeculse later undergo a fur- 

 ther differentiation into a layer next the sporogenous cells, this 

 outer layer constituting the tapetum, and an inner mass of much 

 larger and more colourless cells, the trabecular proper. 



The young tapetal cells do not stain strongly, but later, 

 when they presumably become active in supplying the young 

 spores with food, they stain even more strongly than the spo- 



