686 



ECOLOGY 



arrangement of the" xylem " and " phloem," as in a triarch root. In the Polytri- 

 qhaceae the leaf and stem bundles join, though they are not connected in most 

 mosses. The bundle of Polyirichum certainly is more complex in structure than 

 that of the simpler seed plants. The " vascular bundles " of algae and mosses 



doubtless have no genetic 

 connection with the vascular 

 bundles of higher plants, but 

 they are of great interest as 

 showing possible early steps 

 in the differentiation of con- 

 ductive tracts. 



Variations in primary 

 conductive tissues due to 

 external factors. — Gen- 

 eral remarks. — The vas- 

 cular system often has 

 been regarded as essenti- 

 ally invariable, so far as 

 external conditions are 

 concerned. Recent in- 

 vestigations, hovi^e ver, showr 

 that vascular tissues react 

 to e.xternal changes quite 

 as do other tissues, varia- 

 tions being brought about 

 readily, not alone in the shape, size, and number of the cells or cell 

 fusions, but also in the position of the vascular tract and in the arrange- 

 ment of its members. It is possible in certain cases even to inhibit 

 the development of entire vascular strands, or to stimulate the appear- 

 ance of others in unusual positions. Contrary to earlier views, internal 

 tissues appear to react to external changes about as readily as do such 

 external tissues as the epidermis. 



Water and vascular development. — In amphibious plants different 

 individuals of the same species, or even different parts of the same 

 plant, display more extensive and more specialized conductive tissue 

 in organs exposed to transpiration than in similar organs that are sub- 

 mersed. Similarly, in land plants of a given species the conductive 

 tracts are much better developed in dry air or dry soil than in moist air 

 or moist soil. Thus desiccation, whether brought about by increased 

 transpiration or by diminished absorption, appears to stimulate increased 



Figs. 1013-1016. — The " vascular strand " of 

 the leafy stem of a moss, Polytrichum : 1013, a dia- 

 gram, showing the stem regions, as seen in cross sec- 

 tion; c, cortical region; «, leptome-like cylinder sur- 

 rounding the central hadrome-like strand (A); 1014, 

 cells from the hadrome-like central strand ; note that 

 there are groups of thin-walled cells, suggesting lateral 

 fusion; 1015, cells from the leptome-hke cylinder; 

 note the abundant cell contents; 1016, a longitudinal 

 section through a part of the central strand, showing 

 elongated prosenchymatic cells; all figures highly 

 magnified. 



