222 



The Plant World. 



of Cannon, so that, in the language of the systematist, the appre- 

 ciable features of the root-system doubtless present diagnostic 

 characters quite as marked and as easily recognizable as those 

 of the shoot. 



So long as the gametophyte retained its separate existence, 

 however, and depended upon free water for its reproduction, 

 there could be no real land flora, since the plant must stand, as 

 it were, with one foot in the water. The sporophyte waxed in 

 importance unceasingly, however, by the development of the 

 shoot and root-systems, until its vegetative activity overshad- 

 owed those of the gametophyte, its protective tissues finally en- 

 closing the sexual generation, and with the formation of the 

 pollen tube the seed-plant became wholly able to get away from 

 the stream margin, the low flat moisture-saturated land and to 

 occupy great continental areas and mountain slopes. 



SCARCITY OF FOSSIL REMAINS IN ANCIENT DESERTS. 



The total area of deserts at the present time is equal to 

 about one-sixth of the total land exposure, and undisputed 

 evidence is at hand that extended arid areas existed in all of the 

 great geological periods. Since aridity or humidity depends 

 upon topography, and the prevailing winds, it follows quite 

 naturally that these ancient deserts did not necessarily occupy 

 positions coincident with the deserts of the present. Thus the 

 facts seem to indicate extensive desiccation in what is now 

 eastern America and parts of Europe in the Lower Carboniferous 

 and Permian, while some coincidence of locality is found in arid 

 areas in Wyoming and Texas. While evidence is accumulating 

 to show that great swings or variations in climate are in progress 

 in various regions, yet it would be difficult to demonstrate the 

 proportion of\rid area to^the total land area in any previous 

 period and thus give a quantitative basis for the conclusion that 

 the earth's surface is undergoing progressive desiccation. 



It is noteworthy that formations which give evidence of 

 desert conditions are notably free from fossil plants and contain 

 but little in the way of animal remains, At the time of these 

 earlier periods of aridity, vegetation had not developed forms 

 capable of occupying dry land. During the Carboniferous, how- 

 ever, great areas of low-lying land existed in which conditions 



