STEMS 



687 



vascular development. In_ dry cultures the vessels are more numerous, 

 larger, longer, and have thicker walls than in moist cultures; some of 

 the smalfer veins present in the dry cultures are absent in the moist 

 cultures, remaining as undifferentiated parenchyma. Furthermore, in 

 the drier cultures lignification begins earlier and is much more pro- 

 nounced, and the differential thickening of the walls is more conspicuous; 

 also the endodermis, which 

 often retains its cellulose walls 

 in water or in moist air, has 

 thicker and more completely 

 suberized walls. Finally, the 

 ^hadrome elements die much 

 sooner in dry than in moist 

 cultures. Where growth and 

 transpiration are pronounced 

 from the outset (as in bulbous 

 plants), the first new vessels 

 often are larger than where 

 growth is slow (as in many 

 seedlings) . 



Observation tends to con- 

 firm experiment regarding the 

 influence of water upon vas- 

 cular development. In sub- 

 mersed hydrophytes, such as 

 Elodea and Ceralophyllum, 

 the vascular elements occupy 

 a much smaller space and are 

 much Less differentiated than 

 in land plants of similar size, 

 the leaf bundles often being so small as easily to escape detection (figs. 

 1017, 1018). Although the duckweeds are regarded as vascular plants, 

 their conductive tissues are much less developed than are those of mosses 

 like Polytrichum, entire organs sometimes having no vascular tissue, as 

 in the roots of Lemna. In hydrophytes the leptb'me generally is reduced 

 less than the hadrome, though in rare cases there may be but a single 

 row of sieve tubes within a bundle. In contrast to hydrophytes, most 

 xerpphytes and alpine plants have highly developed conductive systems 

 with large thick-walled elements, and an endodermis that is strongly 



Fig. 1017. A cross section through the vascu- 

 lar bundle of a stem of the waterweed (Elodea 

 canadensis); note that the vascular tract (v~) is 

 not obviously differentiated into leptome and 

 hadrome, and that the vascular cells have thinner 

 walls than the cortical cells (c) ; s, starch grains ; 

 a, intercellular air chamber within the vascular 

 tract; a', similar chambers in the cortex; highly 

 magnified. 



