THE MATERIAL OUTGO OF PLANTS 347 



the age of the tree. But annual rings may show subordinate rings, due 

 to some pronounced climatic change which affected the rate of growth 

 more than once in the year. These rings may be so pronounced as to 

 make the age estimate uncertain, but in temperate regions the annual 

 rings are usually well defined. In some trees the differences between 

 spring and autumn wood are slight, and the annual rings are discerned 

 with more difficulty. The definite annual rings are responsible in large 

 part for the " grain " of wood. (See also Part III on annual rings.) 



Heart wood and sap wood. — "With age the xylem loses its capacity 

 to conduct water, and sooner or later may so change in color and com- 

 position as to distinguish the older heart wood from the newer sap wood. 

 These changes, however, do not coincide with the annual rings, nor do 

 they exactly correspond with the differences in conductivity, since in 

 some plants the whole of the sap wood, but in others only the youngest 

 portion of it, is traversed by the transpiration stream. 



Xylem is water path. — The evidence that the xylem is the path of the 

 transpiration stream rests in part upon direct observation, but mainly 

 upon inference from the effects of cutting the xylem strands or blocking 

 the tracheae. 



Relative development. — In the first place, one finds a general relation 

 between the amount of transpiration and the development of the xylem. 

 In most submersed water plants the xylem is very poorly differentiated, 

 its place being occupied by some elongated cells, slightly different from 

 their neighbors, which are morphologically equivalent to xylem, but 

 physiologically they are negligible. On the other hand, in climbing 

 plants, whose spread of foliage is large and their stems slender, the xylem 

 reaches its best development, occupying a large proportion of the cross 

 section of the stem, and having ducts of relatively large diameter. Not 

 much reliance could be placed upon such a loose and general relation, 

 were it not for more direct evidence. 



Girdling. — Girdling experiments show more clearly the path of the 

 water. It is a matter of common knowledge that by cutting through 

 the sap wood of a tree the foliage promptly wilts and dies; and in 

 earlier days it was commoner than now to see the trees in some piece of 

 woodland " girdled," preparatory to clearing the ground for cultivation. 

 But removing only the bark does not produce wilting, except after weeks 

 or months, for thus only the phloem strands are interrupted. More 

 exact experiments may be performed. By selecting a herbaceous plant 

 whose vascular bundles are distinct, one may cut the pith, the vascular 



