20 MOSSES AND FERNS chap. 



(exospore, exine) strongly cutinised and usually having upon 

 the outside characteristic thickenings, ridges, folds, spines, etc. 

 Where these thickenings are formed from the outside they 

 constitute the third coat (perinium, epispore). The exospore 

 is especially well developed in species where the spores are 

 exposed to great heat or dryness, and which do not germinate 

 at once. In those species that are found in cooler and moister 

 situations, especially where the spores germinate at once, the 

 exospore is frequently thin. The nucleus of the ripe spore is 

 usually small. The cytoplasm is filled with granules, mostly 

 albuminous in nature, with some starch and generally a great 

 deal of fatty oil that renders the contents of the fresh spore 

 very turbid. Some forms, especially the foliose Junger- 

 manniaceae, have also numerous chloroplasts, but this is lacking 

 usually in those forms that require a period of rest before 

 germination. In Pellia and Conocephalus the first divisions in 

 the germinating spore take place while the spores are still 

 within the sporogonium. 



The germination of the spores begins usually by the forma- 

 tion of a long tube (germ-tube, " Keimschlauch " of German 

 authors), into which pass the granular contents of the spore. 

 At the same time there may be formed a root-hair growing in 

 a direction opposite to that of the germinal tube, although quite 

 as often the formation of the first root-hair does not take place 

 until a later period. If the spore does not contain chlorophyll 

 before germination, it is developed at an early stage, before any 

 cell-divisions occur. Often the formation of a germ-tube is 

 suppressed and a cell surface or cell mass is formed at once, 

 and all these forms may occur in the same species. The 

 germination only takes place when the light is of sufficient 

 intensity, and the amount of light is a very important factor 

 in determining the form of the young plant. Thus if the light 

 is deficient, the germ-tube becomes excessively long and slender, 

 and divisions may be entirely suppressed. An excess of light 

 tends to the development at once of a cell surface or cell mass. 

 In the simpler thalloid forms the first few divisions in the 

 young plant establish the apical cell, and we cannot properly 

 speak of the gametophore as arising secondarily from a 

 protonema ; in other cases, however, the young plant does arise 

 as an outgrowth or bud from a protonema, which only rarely 

 has the branching filamentous character of the Moss protonema. 



