510 



LECTURE XXX. 



Experience teaches that even a very thin layer of tissue containing chlorophyll 

 completely uses up all those rays of light which effect assimilation. A thick layer of 

 chlorophyll-tissue has therefore no meaning whatever : in fact it would be a waste 

 of material' in the plant. Accordingly, we find that everywhere in the vegetable 

 kingdom only very thin layers of green assimilatory tissue come to be employed — 

 layers of one or a few tenths of a millimeter thick — even when, as in the case of. 

 succulent plants, the leaves or soft shoot-axes are very thick and massive, in which 

 cases the thin layer of green tissue lies as near as possible to the surface of the 

 organ, in order the better to make use of the incident light. 



It is, on the contrary, of the greatest importance for energetic assimilation, or 

 the production of substances capable of promoting growth, that the thin layers of 

 green tissue should form surfaces as extensive as possible, if the construction of a 

 vigorous growing plant is to be at all accomplished. 



These reflections then show why, as the organisation of the plant gradually 

 approaches perfection from its first beginnings, it is above all necessary to produce 

 organs which are very thin, and possess the largest possible surface of tissue containing, 

 chlorophyll. In the lower Algae this is attained by their assuming the form of thin 

 and long hair-like filaments, or else that of thin ilat lamellae, so that in both cases the 

 volume of the body remains very small, as compared with its surface. These two 

 forms, where the entire plant is thin and filamentous and usually much branched, 

 or else flat and leaf-like, is found again not merely among the highly-organised 

 Algse, but also among the Mosses, and even in some Vascular plants — such as the 

 leafless shrubs Psilotum, Spartium, and others. Here the principle is ever main- 

 tained that only a very thin peripheral layer of assimilatory tissue, accessible to the 

 light, exists. But the purpose mentioned- is attained much more completely when 

 the shoots become differentiated into leaves and axial portions, as is the tendency 

 frequently enough even among the Algae, and almost universally among the Mosses 

 and Vascular plants. By this means it is possible for the shoot-system to present to 

 the light (and thus to the nutritive process) a large number of thin lamellae containing 

 chlorophyll and at suitable distances from one another; and only with such a 

 differentiation into a support (shoot-axis) and lamellae containing chlorophyll (leaves) 

 springing from it, does vegetation in general attain to its higher stages of organisation, 

 and especially to the massive forms inhabiting the dry land, as known to us in the 

 large Ferns, Palms, Conifers, Forest trees and dicotyledonous shrubs. How 

 otherwise could the problem be solved so to construct and support a sheet of 

 assimilatory tissue scarcely 0*2 — 0*3 mm. thick and often many square meters in 

 area, such as is met with in the crown of a Beech or an Oak with its thousands of 

 leaves, or in the few but large leaves of a Banana or Palm ? If we imagine a plant, 

 for instance, of such a kind as Marchantia, where the green shoot itself bears the 

 thin sheet of assimilatory tissue, as large and heavy as a huge Palm or Oak, then it is 

 obvious what a monstrous organisation we should have, and how contradictory it 

 would be to the higher types of the vegetable kingdom. Of course leaves are 

 wanting to plants hke the CacH also : they are satisfied with a thin layer of 

 chlorophyll at the periphery of the thick shoot, and hence their increase in organic 

 mass is relatively very slow ; and, what is perhaps more remarkable, their leafless 

 green shoot-axes assume, even frequently, forms similar to those of leaves— e.g. 



