244 PART III. VEGETABLE PHYSIOLOGY. 



growth ceases, and if a comparison be made between the dry- 

 weight of the tuber at the beginning, and that of the sprouts 

 and exhausted tuber at the close of the experiment, it will be 

 found that there has been no increase ; no new material has been 

 added. Moreover, in plants thus growing in darkness, chloro- 

 phyll very rarely develops ; both leaves and stem have a blanched 

 appearance instead of the normal green. 



It has been found that the part of the spectrum which has 

 the greatest influence on assimilation is that which has the 

 greatest illuminating power, namely, the yellow, but white light 

 is more effective than any one of its component colors. 



Light, also, by influencing the growth of the plants, or by the 

 stimulant effects it exerts upon the living matter of their cells, 

 gives rise to certain movements; but these may be more appro- 

 priately considered under the subject of the movements of plants, 

 which will be taken up farther on. 



Parasites and Saprophytes. It is evident from all that 

 has been said, that chlorophylless plants, including saprophytes, 

 which derive their sustenance solely from decaying organic 

 matter, and those parasites which obtain all their food from living 

 organisms, are to be regarded as destroyers, rather than builders. 

 They are like animals in this respect. Their energy is derived 

 from the tearing down of what others have built up. The line 

 which separates such organisms from ordinary green plants is, 

 however, not always sharply drawn. There are various degrees 

 of saprophytism and parasitism. Many green plants are partly 

 saprophytic, since they do not thrive except when well manured, 

 or abundantly supplied with decaying organic matter. Insect- 

 ivorous plants, like Sarracenia and Darlingtonia, must be regarded 

 in the same light. 



Many plants, like the Dodder, are completely parasitic ; they 

 develop no foliage leaves and no chlorophyll, and they draw 

 their sustenance exclusively from the elaborated juices of other 

 plants. Plants like the Mistletoe, however, are only partially 

 parasitic, for, though this plant grows on other plants, sending 

 its roots into their tissues and absorbing their juices, its leaves 

 contain chlorophyll, and it, no doubt, assimilates some carbon- 

 dioxide on its own account. Moreover, most green plants consist 

 partly of chlorophyll-bearing cells, and partly of chlorophylless 



