FACTORS OF ORGANIC EVOLUTION. 469 



the plant body became more and more elongated. While the plant 

 undoubtedly still absorbed food throughout its entire periphery, it 

 nevertheless began to differentiate into organs. The area chiefly con- 

 cerned in food gathering became broadened into a thallus, a constricted 

 or stem-like portion tended to develop below, and the entire structure 

 anchored itself to the rock by a holdfast or grapple. This holdfast, 

 or so-called root, of most of our present seaweeds is chiefly a means of 

 holding the X3lant in place, and it probably absorbs very little food. 

 As plants emerged into amphibian life, however, the foliar portion was 

 less and less thrown into contact with food, and there was more and 

 more demand upon the grapple which was anchored in the soil. The 

 foliage gradually developed into organs for absorbing gases and the 

 root was forced to absorb the liquids which the plant needed. I do not 

 mean to say that there is any genetic connection between the seaweeds 

 and the higher plants, or that the roots of the two are homologous; 

 but to simply state the fact that, in point of time, the holdfast root 

 developed before the feeding root did, and that this change was plainly 

 one of adaptation. Specialized forms of flowering plants which inhabit 

 water still show a root system which is little more than an anchor, and 

 the foliage actively absorbs water. The same environmental circum- 

 stances are thus seen to have developed organs of similar physiolog- 

 ical character in widely remote times and in diverse lines of the plant 

 evolution. "As the soil slowly became thicker and thicker," writes 

 King in his book upon The Soil, "as its water-holding power increased, 

 as the soluble plant food became more abundant, and as the winds and 

 the rains covered at times with soil portions of the purely superficial 

 and aerial early plants, the days of sunshine between passing showers, 

 and the weeks of drought intervening between periods of rain, became 

 the occasions for utilizing the moisture which the soil had held back 

 from the sea. These conditions, coupled with the universal tendency 

 of life to make the most of its surroundings, appear to have induced 

 the evolution of absorbing elongations, which, by slow degrees and 

 centuries of repetition, come to be the true roots of plants as we now 

 know them." Some aquatic flowering plants are, as we have seen, still 

 practically rootless, and they absorb the greater part of their food 

 directly by the foliar parts; but the larger number of the higher plants 

 absorb their mineral food by means of what has come to be a subter- 

 ranean feeding organ and the foliar parts have developed into gas- 

 absorbing organs and they take in water only when forced to do so 

 under stress of circumstances. 



But as a mere feeding organ the root requires no fibrous structure. 

 It is still a holdfast or grapple and its mechanical tissue has developed 

 enormously, along with that of the stem, in order to preserve the plant 

 against the strain of the moving elements and to maintain its erectness 

 in aerial life. When this self poised epoch arrives, the vegetable world 

 begins its definite and steady ascent in centrogenic form. While the 



