CIRCULATION OF SAP. 99 



in the dead tissues of plants, but with less force than in 

 living ones. 



^ plants that have jio distinct leaves, like most of the 

 Cactuses and Stapelias, tjie epidermis of the stems per- 

 fprms~tne functions of true leaves in other kinds of plants, 

 but just what these functions are has not as yet been 

 fully determined, although many plausible theories of 

 leaf action have been given by vegetable physiologists and 

 chemists, but as they do not agree we may safely conclude 

 that there is not only some mystery surrounding this 

 matter, but that there is still something to be learned 

 about leaf action as well as the chemical changes which 

 take place during the process of assimilation. Plantp, 



animals, to a certain degree posses^ an inherent^owe/ * 

 apitmg themselves to varying conditions, not being M 



controlled ^By' such Tnvariabie laws that their lives are 

 jeopardized by every change of temperature, hygromet- 

 rical condition of the earth and, air, or variations of light , 

 and shade. It can be readily demonstrated that sun- 

 light is necessary for the production of chlorophyll, or 

 the green, coloring matter in the leaves, but it is no 

 more so than for the depositing or production of other 

 natural colors of the leaves, and there are no good reasons 

 for supposing that the green matter in the leaf of the 

 common Beech tree is of any more importance or 

 obtained from a different source than the r^d .pigment in 

 the Igaf of the Purple 5h, or the colors in the leaves of 

 Coleiis^efscTiaffeUii. Neither will it be produced in the 

 absence of light ; still, the 4eeej&t greert in the foliage of 

 plants in general is not to be founo. in countries where 

 there is the most sunlight, but in those where.there is 

 Alternate light an 4 shade, w^itlj, a decided preponderance 

 of the latter. In the deep shade of our forests the 

 Kalmias, Rhododendrons and Hollies not only thrive 

 best, but it is in such situations their leaves assume the 

 deepest shade of green. Aiternate^junlight^and shade, 



