THE LTFE-HISTOEY OF PLANTS. 



219. 



whicli we may assume to resemble cold-blooded 

 animals, requiring far less full exposure to light 

 than others. This is a matter which the gardener 

 finds out by rule of thumb, but it is certain that an 

 investigation of the microscopical structure of leaves 

 would in many cases afford him a sure basis of fact 

 for his guidance. The structure of the leaves of 

 Orchids has imi)ressed this conviction strongly on 

 the mind of the x^i'esent writer. May it not be long 

 ere those who have the requisite leisure and means 

 will work out this question, . so important to the 

 practical cultivator ! 



The changes we have alluded to are not solely de- 

 pendent on light, but also on heat, there being a 

 degree below or above which the changes in question 

 do not take place, and between the two extremes one 

 imder which the change is carried out under the 

 most favourable circumstances. These circum- 

 stances vary in the case of indi\T.dual plants, and 

 although, for practical purposes, they may be often 

 guessed at, yet experience and experiment alone can 

 determine them with any degree of certainty. 



Although we cannot in this place attempt to enter 

 into detail on ' such a matter, we may say that the 

 chlorophyll, which till lately was supposed to be the 

 prime agent in the decomposition of carbonic acid, 

 the elimination of oxygen, and the changes conse- 

 quent upon them, is now by some considered not 

 itself to be the main factor in the series of changes 

 which go on in the leaf when exposed to light. 

 Pringsheim finds in chlorophyll a substance called 

 hypochlorin, which he considers to be an imme- 

 diate derivative from carbonic acid, and the only 

 one which cannot be formed in the absence of light. 

 Chlorophyll itself, he says, is not decomposed during 

 the assimilation of carbon, and is independent of the 

 absorption and even of the presence of carbonic acid. 

 Its decomposition is effected by the absorption of 

 oxygen in respiration. If tliis absorption of oxygen 

 in the process of breathing be carried too far, the 

 chlorophyll is destroyed. If the balance between 

 oxidation as carried on constantly in the breathing- 

 process, and de-oxidation as carried on in the green 

 tissues under the influence of sunlight be upset, and 

 oxidation be carried on disproportionately to de- 

 oxidation, then the plant suffers. According to 

 Pringsheim, it is the green colouring matter, the 

 chlorophyll, which regulates the two. By absorb- 

 ing, as it does, the chemical rays of light, it lessens 

 the energy of the breathing process, and it is owing 

 to the protecting screen afforded by the chlorophyll 

 that the amount of carbon received into and stored 

 in the plant in daylight is greater than the quantity 

 consumed by the respiratory process. The decom- 

 position of carbonic acid, on this view, would be 

 effected by the protoplasm. The question affords an 



additional reason for the microscopical study of the 

 leaf, with a view to obtain hints for the rational cul- 

 tivation of plants deduced from their anatomical 

 structure, such as the presence and thickness of the 

 palisade cells, and its relation to the chemical 

 changes that go on in the leaf. 



The Leaf as a Factory.— Whatever be the 

 exact work done by the chlorophyll, its presence 

 seems essential to the preparation of the food and to 

 the construction of new tissues, and we must briefly 

 allude now to its work in these directions. The 

 substances absorbed by the roots and leaves, acted on 

 by solar light, supply all the materials requisite for the 

 formation of new cell-walls, new protoplasm, and new 

 cell- contents of all kinds. These substances, how- 

 ever, are not all direct productions from chlorophyll, 

 but result from transmutations of one substance into 

 another, and from the reciprocal action of one upon 

 another. The exact nature and sequence of these 

 changes is at present very imperfectly known, 

 though of the first consequence in enabling us to 

 understand what goes on in the living plant. "We 

 can here only allude to some of the better-known 

 phenomena. 



Starch. — The first product in the chlorophyll 

 cells exposed to light is generally admitted to be 

 starch ; some substance chemically allied, to it, such 

 as sugar ; or some fatty matter. Starch, as we 

 have already shown, exists largely in the tissues 

 of most plants, especially in the reserve organs. Its 

 presence is easily ascertained by the aid of the micro- 

 scope, or by soaking the leaves in alcohol to remove 

 the chlorophyll, and then immersing the colourless 

 leaves in a strong solution of iodine, which turns the 

 starch blue. By these means it has been ascertained 

 that the same leaf may contain very different quanti- 

 ties of starch at different periods of the day, or ac- 

 cording to the state of the weather, the increase or 

 decrease being very raj)id. In the dark, or when a 

 portion of the leaf is protected by an opaque body 

 from the light, the starch disappears. Moreover, the 

 starch formed in the day-time under the influence of 

 the light disappears from the leaves at night. This 

 depends on the degree of temperature, and on the 

 health of the jjlant. It may be proved by dividing 

 a growing leaf lengthwise into two halves, at night- 

 fall, after a bright day. One half may be removed, 

 the other half allowed to remain during the night. 

 The half-leaf removed, if at once tested by iodine as 

 above described, will be found full of starch ; the re- 

 maining half -leaf , if tested early next morning, after 

 having been exposed to darkness, will be found to 

 contain very little. 



Again, if the leaf be cut in halves at sunrise no 

 starch is found in the half -leaf that is cut away ; if 



