GEOGRAPHICAL BOTANY AS THE RESULT OF ADAPTATION. 415 



wax as well, and very frequently covered with hair ; this is often dense, 

 making a felt-like layer. The stornata, or breathing pores, are sunk 

 in depressions, themselves being covered by hairs, &c, in various ways. 



All these adaptations to drought have long been noticed ; but the 

 question was, How did they arise ? 



M. Eberhardt has lately experimented upon a number of plants, 

 growing them in ordinary air, in saturated, and in very dry air respec- 

 tively. The results were that plants produced all the above features 

 characteristic of plants growing naturally in our own climate, in deserts, 

 and in a very moist atmosphere. 



He has, therefore, proved experimentally that such adaptations arise 

 in nature by direct response to the action of the surrounding conditions, 

 a fact previously inferred by ample inductive evidence. 



Aquatic Plants. 



In opposition to growing in the driest deserts, many plants are aquatic, 

 and live partly or entirely submerged. 



Here, as before, the forms and structures of the various organs are in 

 strict adaptation or else merely the result of response only. The latter 

 seems to prevail, because the general results are more of the nature of 

 degradations than possessing any advantages to the plant. This is well 

 seen in the feeble development of all kinds of supportive or mechanical 

 tissues, the absence of an epidermis and stornata, &c. 



Again, numerous dicotyledonous plants have dissected leaves when 

 entirely submerged, and, as they belong to many distinct families having 

 no affinities among them, it becomes evident that the details of structure 

 are the result of response to the direct degenerative influence of the 

 water. 



That this is the fact has been proved experimentally to be the case ; for 

 experiments with a plant having dissected leaves below water and completely 

 formed ones in air, bore similar complete leaves under water, when, by 

 dissolving certain salts in it, the increased density of the external medium 

 caused the superfluous water to be withdrawn by osmotic action from the 

 protoplasm of the stems. These then bore, as stated, completely formed 

 leaves under water, similar to those normally produced in air. 



On the other hand, the lacuna or air-passages in the stems, petioles, 

 &c, are increased in number, size, and assume definite forms. These have 

 a distinct use, not only in containing air which assists in supporting the 

 stem under water, but for the accumulation and exchange of gases. 



The following are special results in aquatic plants, showing relative 

 degeneracy, &c. : Leaves have elongated petioles to reach the light. They 

 may be phyl lodes only. The blade may become reduced to filiform 

 segments. A true epidermis with a cuticle is wanting, likewise stornata. 

 The mesophyll is much reduced, and the palisade tissue is wanting. The 

 cortex of the stem may be increased as well as the lacuna, while the fibro- 

 vascular strands are displaced towards the centre ; all supportive tissues 

 are much reduced or wanting. These features are more or less common 

 to all, hence they must have been acquired in response to the aquatic 

 medium. 



