THIN ASSIMILATORY SURFACES. ,511 



Phyllocactus, many species of Opuntia, &c. This occurs in a still higher degree in 

 shrubby plants furnished with Cladodes, as Ruscus, Phyllanthus, &c. 



The vegetable kingdom, so far as it is self-nourished by means of green tissue, is 

 solely and entirely, so far as form is concerned, ruled by the principle of developing 

 on relatively thin supports, or shoot-axes, the greatest possible number of green 

 organs with the thinnest and largest surface possible. The exceedingly graceful 

 growth of plants which contain chlorophyll, so familiar to all, is thus due simply to 

 their containing chlorophyll, because the activity of the assimilating parenchyma only 

 comes into full effect in this case. The contrast presents itself to us at once in the 

 case of plants devoid of chlorophyll, the fructifications of Fungi, and parasitic and 

 saprophytic Phanerogams. It is simply the want of chlorophyll which here makes 

 altogether superfluous the extension of the surface in the form of large leaves ; the 

 body of the plant is therefore developed chiefly as shoot-axes and appears naked, 

 stout, massive, and ungraceful. To them also, as to all and even highly organised 

 water-plants, the formation of true wood is wanting, because they do not need it : 

 since in plants devoid of chlorophyll (which moreover never attain to the size and 

 massiveness of a tree, or even of a large shrub, but usually remain small and incon- 

 spicuous) the want of large leaf-surfaces also entails that of organs of transpiration, 

 and thus the current of water flowing in woody plants from the root to the leaf- 

 crown is dispensed with — they do not require wood because they possess no leaves 

 containing chlorophyll. Moreover, true water-plants are devoid of wood for 

 physiological reasons ; it is true they often possess very large leaf-surfaces, but these 

 are submerged, or they float on the surface of the water, and can themselves take up 

 from the environment the water which they require, and thus do not need an active 

 current of water coming from the roots. We may thus say simply that the formation 

 of wood and (as must be added from what has been previously stated) the formation 

 of numerous thick strands of sclerenchyma is due to the fact that the shoot-axes 

 of terrestrial plants are elevated above the soil, extend their leaf-surfaces to the air 

 and to the light, and find in the lignified tissues of the stems and branches not only 

 elastic and rigid supports, but at the same time the organs which convey to them 

 from the roots water provided with nutritive materials. I have already attempted to 

 make clear by what arrangements the current of water ascending in the wood is 

 conveyed directly into the transpiring green leaves, and how, by means of the 

 venation or distribution of the vascular bundles in these, this water flows into 

 thousands of fine canals, from which the chlorophyll-cells of the leaves take up their 

 nutrient water. I have already shown, moreover (p. 50), how as the size of the 

 leaf-area increases, and thus the extent of the chlorophyll-tissue, the additional and 

 important function falls to the venation of keeping the exceedingly thin lamella of the 

 leaf extended flat, and at the same time of protecting it from lateral rupture. 



This consideration, which it is true only brings out the more obvious 

 features of the organisation of plants, is always traversed by the thought — a guiding 

 thread as it were — that all these arrangements are intelligible and full of purpose 

 only because the plants concerned are nourished by the chlorophyll under the 

 influence of light. 



If however we turn again to the current of water ascending in the wood and 

 distributing itself in the veins of the leaves, we know already that it has fundamentally 



