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 denned. 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 
