102 ACTION OF LIGHT AND HEAT 



Humboldt met with some green plants growing in com- 

 plete darkness, at the bottom of one of the mines of Freu- 

 berg. The mine abounded with hydrogen. Now, wheth- 

 er this gas be endowed with the power of developing the 

 green color, or whether it may enable them to decompose 

 carbonic acid without the aid of light, is a problem which 

 we must leave to more able chemists to resolve. What 

 increases the difficulty is, that carburetted hydrogen gas is 

 a poison no less deleterious to plants than it is to animals. 



Another effect of intensity of light, is to render plants 

 remarkably firm and hard of texture, owing both to the 

 accumulation of carbon, a body of a compact solid nature, 

 and to the increased vigor of their powers of absorption, 

 which enables them to incorporate a greater quantity of 

 the earthy matter dissolved or floating in the increased 

 quantity of sap they suck up. 



Caroline. But, on the other hand, they must contain a 

 greater quantity of liquid, which could produce a contra- 

 ry effect. 



Mrs. B. Recollect that intensity of light increases the 

 power of evaporation, as well as that of absorption ; the 

 plant, therefore, retains no superabundance of liquid, al- 

 though it acquires more of the solid particles which it held 

 in solution. The compactness and hardness of plants ex- 

 posed to an excess of light, offer some impediment to 

 their growth ; their vessels, deficient in elasticity and flex- 

 ibility, are not so susceptible of being elongated by the 

 fluids which circulate within them. Mountainous plants 

 are therefore more diminutive in size than those of the 

 plain. 



Emily. But this is far from being the consequence of 

 exposure to light in hot climates, where the vigor of veg- 

 etation greatly surpasses that of our more moderate tem- 

 perature, and where plants are in general of much larger 

 dimensions than with us. 



Mrs. B. It is to the intensity of heat, rather than of 

 light, that they owe their superiority. When these two 



568. What extraordinary discovery was made by Humboldt'? 569. 

 What is another effect of intensity of light on plants'? 570. What 

 objection does Caroline make to the theory of Mrs. B. that the hardness 

 of plants is increased by intensity of light 1 ? 571. How does Mrs. B. 

 reply to this objectionl 572". Why are mountain plants diminutive 

 in size 1 ? 573. What objection does Emily make to the reason assign- 

 ed by Mrs. B. for the diminutive size of mountain plants'? 



