INFLUENCE OF LIGHT ON STRUCTUEE 485 



air and bears its branches and leaves ; the latter are consequently 

 surrounded by air containing a varying amount of aqueous 

 vapour. The aerial portion is subjected to the alternation of da^y 

 and night, and during these it meets with considerable varia- 

 tions of temperature as well as moisture ; the whole plant is 

 constantly acted upon by the force of gravity. The subterranean 

 parts also find considerable differences from time to time in the 

 temperature of the soil, the amount of moistnre which it 

 contains, and the scanty amount of light which penetrates into 

 the crevices between its particles. The environment, though to 

 a certain extent constant, is nevertheless continually varying in 

 these respects. Particular plants are also subject to other 

 disturbances which are more special in their nature. 



In considering the ways in which these various factors in- 

 fluence the behaviour of plants, we may study first their 

 general relations to light of varying intensity', reaching them 

 equally on all sides. We find many degrees of illumination, 

 attaining in some cases an exceptional brilliance, as when a 

 plant is exposed to the rays of a tropical sun. On the other 

 hand we find cases where a plant is growing in almost total 

 darkness. 



If we consider first the latter case, we find such a plant much 

 modified in form and structure. The stem is usually very much 

 elongated and remains slender ; it is more succulent than the 

 normal one, and bears extremely small leaves which grow out 

 from it at a more acute angle than those which it bears when 

 light has access to it. In cases where the stem is normally 

 very small and the leaves are large and broad, the change in 

 form is different. The stem is but little changed, but the leaves 

 become much elongated and narrow. Certain phjdloclades, 

 such as those of some of the Cacti, become elongated and slender 

 instead of being broad and leaf-like. The structure, too, is 

 modified ; the woody and sclerenchymatous elements are much 

 reduced, and the parenchyma of the cortex is increased in bulk. 

 It becomes more succulent, and the reaction of its sap is much 

 more acid. The chloroplasts do not become green, the pigment 

 which they contain, known as etiolin, being a pale yellow. 

 In the leaves the differentiation of the mesophyU into palisade 

 and spongy parenchyma does not take place. The parenchyma- 

 tous cells of the ground tissue of the stem usually become 

 considerably elongated ; they have, indeed, grown much more 

 than when the plant receives the normal amount of illumination. 

 Plants thus affected by darkness are said to be etiolated. 



