STEMS 689 



is a much greater development of vascular tissue than when they grow into scales 

 in soil or darkness. When seedlings are deprived of leaves, or when they have 

 smaller leaves than usual, or are otherwise poorly nourished, the bundles are 

 smaller than when the seedlings are well-nourished. 



So far as known, changed conditions cause much less variation in leptome than 

 in hadrome. However, an increase in sieve tubes has been observed in Ipomoea 

 and Raphanus grown in solutions of saccharose or glucose ; sometimes in such con- 

 ditions sieve tubes appear even in the hadrome. Remarkable variations have been 

 observed in roots subjected to lateral pressure; for example, in Pisum the primary 

 root, commonly triarch, becomes tetrarch under pressure, and the side roots become 

 polyarch; the commonly pentarch- roots of Vicia Faba similarly become hexarch. 

 In the pine the number of bordered pits increases with the altitude. 



The vascular tissues oj lianas. — The relative area occupied by ihe 

 vascular tract in lianas usually is greater than in other stems of similar 

 proportions, and the individual elements possess unusual length and size ; 

 the Cucurbitaceae with their capacious vessels and sieve tubes fur'nish a 

 familiar illustration. Many woody lianas exhibit peculiar secondary 

 tissues; for example, Bignonia capreolata has radial plates of pihloem 

 penetrating far into the xylem, thus appearing like a cross in section 

 (whence the name cross vine). In Mucuna there are alternating rings of 

 phloem and xylem, and in Rhus Toxicodendron a cross section of the 

 climbing stem is strikingly eccentric, owing to the much greater wood 

 development on the side toward the support. That the characteristic 

 structural features of lianas may be due in part to external factors is 

 clear from the fact that in Vitis vinifera the vascular tract in climbing 

 stems is much more differentiated than in stems that do not climb. 

 However, lianas have been inadequately studied, and little is known 

 concerning the cause or significance of their peculiar secondary tissue. 



Variations in secondary wood due to external factors. — The annual 

 ^ng. — In most trees and shrubs of temperate climates growth is much 

 more vigorous in spring than later, the spring wood being characterized 

 by large thick-walled vessels, and the summer or autumn wood by 

 small thin-walled vessels (fig. 10 ig). The contrast between the spring 

 wood and the autumn wood often is the chief circumstance which 

 makes it easy to discern the growth rings of trees. The theory has 

 been advanced that the decreasing size of vessels from spring to 

 autumn is due to the gradual increase of pressure to which the growing 

 tissues underneath the bark are subjected. A more tenable theory, 

 harmonizing better with conditions in primary conductive tissues, is 

 that the size and number of the cells and the thickness of the walls are 



