41 



groTm^in artificial cultures,- also Hymenomycetes. In mine; 

 and other moist localities without light, deformed mushroom 

 have repeatedly been collected, whose tissue differentia-^ 

 tion remains sub-normal. ■'■ 



(44) In the vascular plants, we will briefly discuss in order the 

 formation of the epidermis, the mesophyll and the conducting 

 and mechanical tissues. 



Epidermis, mesophyll. In cross sections through leaves 

 and stems, one usually finds the epidermis, in so far as the 

 primary dermatogen is still retained, sharply separated from 

 the tissue lying beneath it. Taking no account of differen- 

 ces in size existing, on the one hand, in cells of the epi- 

 dermis, and the other hand, in those of the mesophyll and 

 the primary bark, the distinguishable content of chlorophyll, 

 the characteristic form of the mesophyll cells, the thick- 

 ening of the walls of the bark cells, and many others, may 

 still become important, according to the plant species con- 

 cerned. The difference between the epidermis and the tissue 

 layers lying beneath it, can be eliminated or at least de- 

 creased Since the iniier tissues lose their capacity for char- 

 acteristic formation, as in the needles of Pinus austriaca , 

 illustriated in figure 9, in which the cells of the hypoderm 

 as well as those of the epidermis have remained thin-walled, 

 or as in many dwarf specimens in which there is lacking a 

 formation of mechanical tissue in the bark.^ This can also 

 take place if the cells of the epidermis follow the course 

 of development fehat under normal life conditions falls only 

 to the lot of the more deeply lying layers. Abundant chloro- 

 phyll is developed in submerged epidermal cells, for instance, 

 in the leaves of Sa^ittaria, which are forced to develop 

 belovi^ the surface of the water, ^ 



If the cells of the epidermis are compared with one 

 another a #ery different degree of "division of labor" may 

 be recognized in their formative and functional characters; 

 not infrequently, such a division is lost altogether. In 

 many leaves the epia.ermis of the upper side shov/s a complete- 

 ly homogeneous tissue-plate composed throughout of the same 

 kinds of cells. In the majority of cases elements of dif- 

 ferent kinds take part in the composition. If v/e take no 

 account of those plants in which certain epidermal cells un- 

 dergo especial development as crystal reservoirs, secreting 

 glaMs or idioplasto-containing cystoliths, three especially 



1. Compare for example, v. Bambeka, S. uh exemplaire 

 monstrueux de Polyporus sulfur eus. Bull. Soc. My col. France, 

 1902, T. XVIII, p. 34. 



8. Concerning leaves of the witches brooms on Abies , 

 and the like, compare, l^elow. Chapter V, B, 5. 



3, Compare Costantin, Rech, s. 1. Sagitaire. Bull, Soc. 

 Bot. France, 1885. T. XXXII, p, 818. 



