513 



LECTURE XXX. 



only the one purpose of conveying the nutritive substances of the soil to the 

 assimilating tissue. But these substances are dissolved in very large quantities of 

 water taken up by the roots in the soil, and I have previously shown in detail what^ 

 difficulties the roots of terrestrial plants have to struggle with in order to obtain 

 these large quantities of water from a soil which is scarcely moist, and that they are' 

 even compelled first to dissolve a portion of the nutritive materials by means of their 

 innumerable root-hairs, in order to transfer it to the current of sap. The roots, 

 however, are only adapted for this function when they in their turn obtain not only 

 the necessary surface, but also a sufficient number of points of contact with the 

 particles of the soil. The former is attained in the first place by the great length 

 which the totality of all the root-fibres and their ramifications reach in the soil; by 

 means of this development in length, and the usually very small diameter of the 

 absorbing roots, a relatively large surface extension is already attained, but this is 

 supplemented in an exceedingly advantageous manner by the production of the 

 multitudinous root-hairs, and their continual new formation behind the forward- 

 growing apex of the root. All these properties of the roots, which have already 

 been referred to in the lectures on Organography, are developed only in terrestrial 

 plants which contain chlorophyll : these alone need a root-system so extensive and 

 so organised. We must again perceive that it is to the properties of the chlorophyll 

 and its nutritive activity that the organisation of the roots of terrestrial plants is 

 due. Land-plants devoid of chlorophyll, since they need no transpiration-current, 

 also possess but few roots, and these are short and for the most part thick: 

 water-plants on the contrary are again, for another reason, spared the trouble of 

 developing an extensive root-system ; since they are in a position to take up water 

 and nutritive materials at the whole of theii- surface, or, if the leaves float, at least 

 at their lower surface, no root-system whatever is needed, or at any rate only an 

 accessory one, to maintain the nutritive activity of the chlorophyll. In Algae, and 

 even in some aquatic Phanerogams, the roots are chiefly or it may be exclusively 

 organs of attachment to fix the plant in its position, thus again contributing to keep 

 those parts of the plant which contain chlorophyll in such a position that they 

 can assimilate under the influence of the light. 



Evidently the relations of organisation which are merely sketched here, have 

 arisen by correlations in the progressive development of the vegetable kingdom ; and 

 in fact we can see up to a certain point how this has come to pass, so that the 

 better the chlorophyll-tissue was able to extend itself to the light in thin broad 

 sheets, the more capable it would be of producing large quantities of vegetable 

 substance, which would provide the material for the" formation of the huge masses of 

 wood; and on the other hand we know that the more complete aeration of 

 the soil as the roots of the land-plants need it, directly and essentially contributes 

 to promote the lengthening and branching of the roots. In proportion as this 

 occurs, the roots in the earth are also able to transmit larger quantities of nutritive 

 water to the chlorophyll, which in its turn produces the material not only for the 

 conducting wood-body but also for the growth of the root. 



We can, however, adduce numerous other relations of organisation as necessary 

 correlates of the activity of chlorophyll. The Geotropism of the roots as well as 

 tha^ of the shoot-axes serves (as does also the heliotropic sensitiveness of the latter 



