CONDITIONS OF ABSORPTION 215 



capacity, is not altogether able to exclude substances that are devoid of 

 nutritive value. 



It will be convenient, in considering the conditions under which 

 absorption is carried on, to begin with the case of a submerged green 

 plant such as a unicellular or multicellular Green Alga. Plants of 

 this kind, surrounded as they are on all sides by a medium which 

 contains all the necessary food-constituents in the proper degree of 

 concentration, can absorb nutrient materials over the whole of their 

 outer surface. In these circumstances special absorbing tissues can, as 

 a rule, be dispensed with ; the root-like organs with which many Algae 

 are provided probably never exactly correspond to the roots of land 

 plants, but serve solely as organs of attachment, and thus properly belong 

 to the mechanical system. 



Terrestrial green plants are placed under very different con- 

 ditions. They obtain water and food materials, partly from the 

 soil in which they are rooted, and partly from the atmosphere that 

 envelops their aerial organs. As a rule the air only provides a single 

 one among the food-constituents, namely, carbon dioxide ; but this 

 substance is of the very greatest importance, since it represents the raw 

 material from which the green cells extract carbon by the process of 

 photosynthesis. The relation of a terrestrial plant to atmospheric 

 carbon dioxide is thus comparable to that of a submerged plant to the 

 whole of its food-materials. The photosynthetic organs may absorb 

 carbon dioxide over their entire surface, as is the case among Mosses ; 

 where on the contrary the external surface is insufficient or unsuitable, 

 the defect is remedied by the development of a system of internal air- 

 spaces. The intake of carbon dioxide thus forms part of the more 

 general process of gaseous interchange just like the absorption of 

 respiratory oxygen. No special tissue is known to exist for the 

 purpose of absorbing carbon dioxide or of conveying it in a state of 

 solution to the places where it is utilised. 



From the soil, terrestrial green plants obtain water and the nutrient 

 salts dissolved therein ; for this purpose they develop a system of 

 special absorbing organs or roots ; these structures are further provided 

 with a specialised absorbing tissue, which in a purely topographical and 

 ontogenetic sense corresponds to the epidermis of the aerial organs. 



Modifications of the typical absorbing arrangements are far from 

 uncommon among land plants ; they generally arise in connection with 

 the special mode of life of the plants in question, or, in other words, 

 in relation to certain climatic or edaphic peculiarities of their habitat. 

 The most frequent of these modifications depends upon the water- 

 absorbing capacity possessed by many aerial organs, and particularly 

 by leaves. This mode of absorption plays a prominent part in the 



