8 



xAs the leaves of most plants are expanded m a drier 

 medium than that which surrounds the roots, these pores 

 naturally allow a large amount of moisture to escape par- 

 ticularly in dry weather. If we cover a plant with a bell- 

 jar we can see this moisture condensing on the sides of 

 the glass, and in carefully conducted experiments we can 

 actually measure the amount of water so lost. From such 

 calculations we estimate that a fair-sized tree standing m 

 the open may lose ten gallons of water in the course of 

 a summer's day. This loss of water from the leaves might 

 at first be thought to be an accidental phenomenon due 

 to the possession by plants of delicate expanded leaf struc- 

 tures. As a matter of fact, however, this process of trans- 

 piration is of vital importance to the plant. In the 

 first place it can be shown by experiment that leaf trans- 

 piration exerts a considerable amount of suction, and it is 

 in part by this means that vx^ater is raised in tall trees to 

 the upper branches. In shrubs and herbaceous plants this 

 suction alone would be sufficient to raise the water into the 

 leaves and flowers, though the roots too are equally con- 

 cerned in the ascent of sap. Nor must we consider the 

 water which is given off by the leaves as so much waste. It 

 is indeed important that some of the water absorbed by the 

 roots should be driven off. For the water contained m the 

 soil contains the mineral salts which are necessary for the 

 plant, m very dilute solution, and these salts require there- 

 fore to become concentrated in the plant. This can only 

 be done by driving off some of the superfluous water and 

 that is efi'ected by the process of transpiration. The leaves 

 are therefore acting as a condensing apparatus and thus 

 performing a very needful function. 



I trust that what I have said regarding the sensitive- 

 ness of the various parts of the plant to external forces 

 and surrounding conditions will have impressed the reader 

 with the power of response inherent in plants, and this 

 will indicate how by artificially selecting our medium and 

 method of treatment we can considerably modify the 

 course of development of the plants we cultivate. 



