678 THE POPULAR SCIENCE MONTHLY. 



loon at the gas-works. No increase in size is possible without 

 increase of the internal pressure. Turgescence, as the state of ten- 

 sion is called in plant physiology, depends upon the amount of 

 liquid in the cells, and may be regulated by the protoplasm. In- 

 deed, a leaf may be strangled as readily as an animal which is 

 taken by the throat. If one ties a string tightly around the 

 petiole, water evaporates from the blade, and can not be supplied 

 from below in sufficient quantities to keep the cells tense and 

 elastic. Consequently, the whole leaf relapses into a state of flab- 

 biness, and growth is impossible. Remembering, then, that 

 growth of cells, and consequently of cell-tissue, is an unheard-of 

 thing without turgescence, hydrostatic pressure, or, in a word, 

 stretching, let us see what effect light has upon the condition of 

 things within the plant-cell. 



Although experiments along these lines are difficult to make, 

 and, when made, difficult to interpret, it is the opinion of most 

 botanists that the effect of light upon growth is one of retardation 

 rather than of acceleration. It is true that plants will not thrive 

 in darkness, but that is due to cessation in the assimilating pro- 

 cesses. This is a comparatively clear case, and may be tested by 

 experiment. Let a potato-tuber be cut in halves, each half con- 

 taining one or more buds, or "eyes," and then let one half be 

 allowed to sprout in darkness, while the other is brightly illumi- 

 nated. Conditions of temperature and moisture should be pre- 

 cisely the same in each case. What, then, will be the result ? Sim- 

 ply this : After a certain period, the length of which may vary 

 from a day or two to more than a week, each half of the tuber will 

 sprout, put forth a shoot, and upon this shoot there will be devel- 

 oped leaves. The two shoots will, however, be unlike. The one 

 grown in light, or under normal conditions, will be short, plump, 

 firm, green in color, and will bear well-developed green leaves. 

 The shoot grown in the dark — the etiolated shoot, as it is tech- 

 nically named — will be long and slender, the leaves will be smaller, 

 and in neither leaves nor shoot will there be a healthful green 

 color. In other words, light seems to contribute to the production 

 •^of a shorter shoot, and darkness to a longer. As to the growth 

 of leaves, which are the assimilating organs of the plant, it is 

 natural to suppose that in darkness they would be smaller ; and 

 such, in fact, is shown to be the case by the experiment with the 

 potato. Having less to do, and less to do with, than leaves grown 

 under normal conditions, they are correspondingly smaller and 

 weaker. Microscopic examination of the two shoots will show fur- 

 thermore that the fundamental tissue-cells in the etiolated shoot 

 are much thinner- walled than in the normal shoot. In brief, the 

 cells .are stretched more tightly by the contained protoplasm when 

 free from light than they are when exposed to its influence. 



