INFLUENCE OF LIGHT ON PLANTS. 67 



88. The effect of the complete and continued withdrawal of Light 

 from a growing plant, is to produce an etiolation or blanching of its 

 green surfaces : a loss of weight of the solid parts, owing to the conti- 

 nued disengagement of carbon from its tissues, unbalanced by the fixa- 

 tion of that element from the atmosphere ; a dropsical distension of the 

 tissues, in consequence of the continued absorption of water, which is 

 not got rid of by exhalation ; a want of power to form its peculiar secre- 

 tions, or even to generate new tissues, after the materials previously 

 stored up have been exhausted ; in fine, a cessation of all the operations 

 most necessary to the preservation of the vitality of the structure, of 

 which cessation its death is the inevitable result. A partial withdrawal 

 of the influence of light, however, is frequently used by the Cultivator, 

 as a means of giving an esculent character to certain Plants, which 

 would be otherwise altogether uneatable ; for in this manner their tis- 

 sues are rendered more succulent and less " stringy," whilst their pecu- 

 liar secretions are formed in diminished amount, and communicate an 

 agreeable flavour instead of an unwholesome rankness of taste. 



89. There is one period in the life of the Flowering Plant, however, 

 in which the influence of Light is rather injurious than beneficial ; this 

 is during the first part of the process of germination of seeds, which is 

 decidedly retarded by its agency. This forms no exception, however, 

 to the general rule ; since the decomposition of the carbonic acid of the 

 atmosphere, and the fixation of carbon in the tissues, do not constitute 

 a part of the operation. On the contrary, the embryo being nourished, 

 like an animal, by organic compounds previously elaborated and stored 

 up in the seed, the chemical changes which take place in them involve 

 the opposite action, the extrication of carbon, which is converted into 

 carbonic acid by uniting with the oxygen of the atmosphere. It is 

 obvious, then, why light should not only be useless, but even prejudicial, 

 to this process ; since it tends to fix the carbon in the tissues, which 

 ought to be thrown off. As soon, however, as the cotyledons or seed- 

 leaves are unfolded, the influence of light upon them becomes as impor- 

 tant, as it is on the ordinary leaves at a subsequent time ; their surfaces 

 become green, and the fixation of carbon from the atmosphere com- 

 mences. Up to that point, the young plant is diminishing day by day 

 (like a plant that is undergoing etiolation), in the weight of its solid 

 contents ; although its bulk has increased by the absorption of water. 

 From the time, however, that its cotyledons begin to act upon the air, 

 under the influence of light, the quantity of solid matter begins to in- 

 crease ; and its augmentation subsequently takes place, at a rate pro- 

 portional to the amount of green surface exposed, and the degree of 

 light to which it is subjected, 



90. The influence of Light upon the direction of the growing parts 

 of Plants, upon the opening and closing of flowers, &c., is probably 

 due to its share in the operations already detailed. Thus the green 

 parts of Plants, or those which effect the decomposition of carbonic 

 acid (such as the leaves and stems), have a tendency to grow towards 

 the light ; whilst the roots, through whose dark surfaces carbonic acid 



3 thrown out by respiration, have an equal tendency to avoid it. 

 That the first direction of the stems and roots of plants is very much 



