STEMS 707 



arising from a phellogen layer originating in the callus, is known as wound cork. 

 Probably the incitation to cambial activity is given by the increased flow of material 

 arising from greatly accelerated transpiration, which also favors cork development. 

 The direction of growth of the callus tissues is in the path of least resistance, that 

 is, toward the exterior. When the cork layer of a potato tuber is exposed to strong 

 radial pressure during its development, the chief cell divisions are radial, resulting 

 in strong lateral growth instead of the usual radial growth.' 



The thickness of bark varies considerably with the habitat, being 

 greatest in deserts and other dry situations and in alpine regions, and least 

 in the tropical rain forest. Individuals of a species common to two 

 situations have the thicker bark in the more xerophytic habitat; alpine 

 and light cultures show more bark than lowland and shade cultures. 

 Probably in most cases thick bark is associated with high transpiration 

 and thin bark with low transpiration. The slight development of bark 

 in roots and in rhizomes also is in harmony with this view. Thick 

 bark in submersed stems does not invalidate this view, since increased 

 thickness in such cases is due to the large development of air spaces 

 (as in Decodon, p. 553). 



The role of cork. — The conduction of food is the chief role of living 

 bark, but the dead bark has a protective role of great importance be- 

 cause of the slowness of its exfoliation. The thickness of the bark with 

 its large number of dead air-containing cells contributes to its pro- 

 tective efficiency, but the cork layer is by far the most significant 

 feature. Partly because of the absence of air spaces, but more because 

 of suberization, cork is about the most impermeable of tissues, and thus 

 is of great value in checking transpiration. A potato that loses but 0.04 

 grams in weight in twenty-four hours if unpeeled, loses 2.56 grams if 

 peeled, or sixty-four times as much, and yet the cork layer of a potato 

 is so thin as to appear to the naked eye as a mere film. Twigs with 

 the thinnest of cork layers transpire with almost inappreciable slowness, 

 if the lenticels and cracks are artificially sealed. Probably the chief 

 value of cork is in checking transpiration in periods of relative in- 

 activity, as in seasons of periodic drought or of winter cold, when there 

 is little or no absorption; during such periods the exposed parts of 

 deciduous trees and shrubs are completely mantled by an almost imper- 

 vious coat of cork and bud scales. 



' Similarly, in the spores of Equisetum and in the egg of Fuciis, the usual direction 

 assumed by new cell walls can be changed through the influence of pressure; comparable 

 pressure effects have been reported in the roots of Vicia Faba, but recent work is not 

 confirmatory. 



