Jtdy 19, 1888] 



NA TURE 



271 



Now there are very many foes which injure the leaves 

 of our timber-trees, and I wish to show, as clearly as pos- 

 sible in a short article, how it comes about that injury to 

 the leaves means injury to the timber. The sum total of 

 the matter is that the substances which are to be sent 

 down to the cambium, and converted through its agency 

 into wood, are produced in the cells of the leaves : conse- 

 quently, from our point of view, when an insect or a 

 fungus consumes the substance of the leaves, it consumes 

 timber in prospective. Similarly, when the leaves are re- 

 moved from a tree by any agent whatever, the latter is 

 robbed in advance of timber. A leaf, generally speaking, 

 is an extended, flattened portion of a branch, covered 

 by a continuation of the epidermis of the branch, and 

 containing a continuation of its other tissues — the vascular 

 bundles of the branch being continued as the venation, 

 and the cellular cortex reappearing as the green soft tissue 

 of the leaf. The epidermis of the 'leaf is so pierced at 

 hundreds or thousands of nearly equi-distant points, that 

 gases can enter into or escape from all its tissues : at these 

 points are the so-called stomata, each stoma being a little 

 apparatus which can open and close according to circum- 

 stances. 



These openings lead into excavations or passages 

 between the loose cells of the softer leaf-tissue, and if we 

 supposed a very minute creeping organism to enter one 

 of the stomata, it would find itself in a labyrinth of inter- 

 cellular passages : supposing it able to traverse these, it 

 could pass from any part of the leaf to any other between 

 the cells ; or it could emerge again from the leaf at thou- 

 sands of places — other stomata. In traversing the whole 

 of the labyrinth, however, it would pass over many 

 millions of times its own length Moreover it would 

 find these intercellular passages filled with a varying 

 atmosphere of diffusing gases — oxygen, nitrogen, the 

 vapour of water, and carbon-dioxide being the chief. It 

 would also find the cell-walls which bound the passages 

 damp, with water continuous with the water in the cells. 

 If we suppose our hypothetical traveller threading the 

 mazes of these passages at night, and able to perceive the 

 changes which go on, it would find relatively little oxygen 

 and relatively much carbon-dioxide in the damp atmo- 

 sphere in the passages ; whereas in the daylight, if the sun 

 was shining brightly on the leaves, it would find the atmo- 

 sphere rarer, and relatively little carbon-dioxide present, 

 but an abundance of oxygen. These gases and vapour 

 would be slowly moving in and out at the stomata by 

 diffusion, the evaporation of the watery vapour especially 

 being quicker on a dry, hot, sunny day. 



Inside the cells between which these tortuous passages 

 run, are contained structures which have much to do with 

 these changes. Each of the cells I am considering con- 

 tains a lining of protoplasm, in which a nucleus, and a 

 number of small protoplasmic granules, coloured green, and 

 called chlorophyll corpuscles, are embedded : all these are 

 bathed in a watery cell-sap. 



Now, putting together in a general manner some of the 

 chief facts which we know about this apparatus, it maybe 

 said that the liquid sap inside the cells gives off water to re- 

 place that which escapes through the damp cell-walls, and 

 evaporates into the above-named passages and out through 

 the stomata, or at the surface. This evaporation of the 

 water is in itself the cause of a flow of more water from 

 behind, and this flow takes place from the vascular bundles 

 forming the so-called venation of the leaf, coming directly 

 from the wood of the stem. The course of this water, 

 then, is from the soil, through the roots, up the young 

 wood and into the venation of the leaf, and thence it 

 is drawn into the cells we are considering. But this water 

 is not pure water : it contains in solution small quantities 

 of salts of lime, potash, magnesia, nitric, sulphuric, and 

 phosphoric acids, as well as a little common salt, and traces 

 of one or two other things. It is, in fact, of the nature of 

 ordinary drinking-water, which always contains minute 



quantities of such salts : like drinking-water, it also con- 

 tains gases (oxygen, nitrogen, carbon-dioxide) dissolved 

 in it 



It follows from what has bem said that the cell-sap tends 

 to accumulate small increasing quantities of these salts, 

 &c, as the water passes away by evaporation. But we 

 must remember that the living contents — the protoplasm, 

 nucleus, and the green chlorophyll-corpuscles — use up 

 many of these salts for their life-purposes, and other 

 portions pass into the cell-walls. 



It will thus be seen that the green chlorophyll-corpuscles 

 are bathed by a fluid cell-sap, the dissolved gaseous and 

 mineral contents of which are continually changing, even 

 apart from the alterations which the life-processes of the 

 living contents of the cell themselves entail. We may say 

 that the chlorophyll-corpuscles find at their disposal in the 

 cell-sap, with which they are more or less in direct con- 

 tact, traces of salts, oxygen, carbon-dioxide, and of course 

 water, consisting of hydrogen and oxygen. 



Now we have the best possible reasons for knowing that 

 some such changes as the following occur in these chloro- 

 phyll-corpuscles, provided they are exposed to sunlight : 

 they take up carbon-dioxide and water, and traces of 

 minerals, and by means of a molecular mechanism which 

 is as yet unexplained in detail, they perform the astonish- 

 ing feat — for it represents an astonishing transformation 

 when regarded chemically and physically — of tearing 

 asunder, by the aid of the light, the carbon, hydrogen, and 

 oxygen of the carbon-dioxide and water, and rearranging 

 these elements in part so as to form a much more complex 

 body — starch, or an allied compound, oxygen being at the 

 same time set free. 



It is of course not part of my present task to trace these 

 physiological processes in detail, or to bring forward the 

 experimental evidence on which our knowledge of them 

 is based. It must suffice to state that these compounds, 

 starch and allied substances, do not remain in the chloro- 

 phyll-corpuscles, but become dissolved and carried away 

 through certain channels in the vascular bundles of the 

 venation, and thence pass to wherever they are to be em- 

 ployed as food. The chemical form in which these sub- 

 stances pass from one cell to another in solution is chiefly 

 that of grape-sugar, and it is a comparatively easy observa- 

 tion to make that the cells so often referred to contain 

 such sugar in their sap. 



We are only concerned at present with the fate of a 

 portion — but a very large portion — of this starch and 

 sugar : we can trace them down the vascular bundles of 

 the venation, through the leaf-stalk, into the cortex, and 

 eventually to the cambium-cells ; and it is necessary to be 

 quite clear on the following points : (1) the cambium-cells, 

 like all other living cells which contain no chlorophyll, 

 need to be supplied with such foods as sugar, starch, &c, 

 or they starve and perish ; (2) since these foods are pre- 

 pared, as we have seen, in the leaves, and in the leaves 

 only, it is obvious that the vigour and well-being of the 

 cambium depend on the functional activity of the leaves. 

 We have already seen how the cambium- cells give 

 rise to the young wood, and thus it will be clear how 

 the formation of timber is dependent on the functional 

 activity of the leaves. Moreover, it ought to be mentioned, 

 by the way at least, that it is not only the cambium 

 which depends upon the leaves for its supplies — all the 

 roots, young buds, flowers, and fruits, &c, as well as 

 the cortex and cork-forming tissues, are competitors for 

 the food supply. Now it is clear that if we starve the 

 buds there will be fewer leaves developed in the follow- 

 ing year, and so next year's cambium will again suffer, 

 and so on. 



I have by no means traced all the details of even the 

 first ramifications of the complex network of correlations 

 implied by this competition of the various organs and 

 tissues for the food supplies from the leaves ; but probably 

 the following proposition will be generally clear : — If the 



