10 BULLETIN 1059, U. S. DEPARTMENT OF AGRICULTURE. 
may function with less water loss than one exposed to the full heat- 
ing effect of sunlight. 
Until the distribution of essential substances, such as phosphorus, 
in the plant has been more fully and carefully studied in relation 
to light and the volume of the water stream, it is impossible to 
form a fair opinion as to whether the latter, and the transpiration 
of a large volume of water, are really essential to the end for which 
the plant exists. On this point botanists have ever been at variance. 
For the present, however, transpiration is believed to be merely 
an unavoidable concomitant of carbon-dioxide absorption, serving 
no useful purpose when carried to extremes, while always menacing 
the existence of the plant. 
There is now only one more very essential point to be touched 
upon — a point which is of especial interest in connection with the 
study of trees because of their perennial character. The continu- 
ous absorption of water at the roots and its loss at the leaves of 
plants is necessarily accompanied by the absorption of all salts 
which are contained in the soil solution. There is undoubtedly some 
so-called selective absorption in the sense that any semipermeable 
membrane admits the complex molecules less readily than the simple 
ones, but the ability of the plant to differentiate between useful and 
unnecessary salts is not admitted. It is therefore inevitable that 
the leaves should accumulate quantities of material which can not 
be used: that there should be a tendency for such materials to dif- 
fuse back toward the roots; that when such materia] is present in 
sufficient quantities it should be precipitated or crystallized, and in 
such form should tend to obstruct the flow of water in the channels 
where it exists. It is conceivable, then, that all tissues which are 
actively engaged in the transport of water must eventually become 
" silted up " with this useless material and that this is the cause of 
senescence. Its best illustration is, perhaps, in the petiole of the 
broad leaf, through which narrow passage a large evaporating sur- 
face must be supplied. This conception explains quite well the 
eventual failure of leaves to function and their gradual drying and 
falling, even in those forms in which the leaves are not in the least 
sensith-e to seasonal changes. It also, perhaps, explains the forma- 
tion of heart wood in trees. The more important idea, however, is 
tli at it points to the necessity for growth to maintain existence. It 
is not sufficient that the " suppressed " tree (as every forester calls 
the tree growing with insufficient light) should obtain enough water 
to prevent the desiccation of the foliage. The plant must be peri- 
odically enabled to produce some new growth or it succumbs to 
senility, regardless of age. Apparently the maturity of a normal 
or even a dominant tree is attained soon after its limit of height is 
