ADJUSTMENT TO WATER 39 



more and more into the air. In consequence, such plants have 

 developed the part of the body in contact with the water supj^ly 

 into *a special organ for absorption, i.e., the root. The funda- 

 mental nature of this relation is seen clearly in those terrestrial 

 forms that have come to be submerged. The absorptive organ is 

 lost and the function of absorption is again distri])uted over the 

 entire surface. 



Absorption is an inherent })roperty of protoplasm. In tei-restrial 

 forms, and in flowering plants especially, it is confined to the root, 

 and the form and structure of the latter have an immaliate 

 bearing upon this function. Absorption consists of three processes 

 or factors: imbibition, osmosis, and protoplasmic attraction. 

 These act in vmison, but for the sake of clearness they will first 

 be considered separately, following a preliminary account of the 

 root. 



52. The form of roots. The most primitive terrestrial plants, 

 liverworts, mosses, fern prothallia, and a few algae, such as Botry- 

 dium, possess filamentous roots or rhizoids. The higher terrestrial 

 forms, the ferns and flowering plants, possess massive roots, in 

 which the absorbing surface is regularly modified to form root- 

 hairs. The latter are long, tube-like cells, which correspond in 

 structure and function to the rhizoids of simpler plants. The 

 form of the root is variously modified in response to the character 

 of the habitat, and to the need of storing food material. Such 

 changes are structural responses and are consideral under adapta- 

 tion. The structure of the root, on the other hand, is more or 

 less the same for all flow^ering plants, and a knowledge of it is 

 essential to an understanding of the functions of the root. 



53. Primary regions of the root. Roots, like stems and leaves, 

 consist of three primary regions, which are distinguishable in the 

 embryo, and remain more or less di.stinct throughout tiio life of 

 the plant. These regions when in the condition of primary mcristein 

 are termed dermatogen, periblcm, and j)leromc. The dermatogen 

 or "epidermis producer" is the outermost layer of the ciilim 

 plant body. It normally persists throughout the life of the i)lant 

 as a single layer, owing to the fact that \hv division of its celKs 

 occurs in two planes only. The i)lerome is the central cylinder 

 or core of the plant. It develops primarily into the fil)r()vascular 

 system. Between the dermatogen and the pleromo lies the broad 

 area of the pcriblem, which changes largely h^in tho cortical or 



