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 material 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 

 sensitive to seasonal changes. It also, perhaps, explains the forma- 

 tion of heartwood in trees. The more important idea, however, is 

 that 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 



