48 METABOLISM 



and sieve tubes. That these elements have nothing to do with the transport 

 of water in the stem is shown by a simple experiment, first carried out by Hales 

 (1748) and repeated on numberless occasions since his time and always with 

 the same result. In order to break the continuity two circular incisions are 

 made round the stem right into the wood, and the intervening ring of tissue 

 is removed. If this ' ringing ' be not carried out too extensively, and if due 

 care be taken that the stem does not become dried up or rotten at the region 

 of ringing, the leafy crown will remain fresh for a long time, and the transport 

 of water will not be interrupted to any appreciable extent by the ringing. 

 We may conclude, therefore, that the conduction of the water is effected by the 

 wood. In the long run, of course, it is impossible to prevent injuries to the 

 exposed wood, when its capacity for conducting water will decrease and the 

 leaf-bearing region above the ring dies after a few years, unless it has contrived 

 meanwhile to make itself independent of the original root-system by the forma- 

 tion of a special new root-system above the excised ring. Observations made by 

 Trecul (1855) have shown how long a tree can, in spite of such treatment, re- 

 main alive above the point of ringing. A lime tree at Fontainbleau, for example, 

 showed signs of life in its apex forty years after ringing had been performed. 

 From an examination of amputated branches, which, as every one knows, 

 remain for a long time fresh, and are therefore capable of transporting water, 

 one is easily able to conclude, by the method of exclusion, that the carriage 

 of water takes place only by means of the woody tissue. If at the lower end 

 a branch be cut in such a way that only the cortex, the pith, or the wood 

 comes in contact with water, it will be found that in the first two cases the twig 

 rapidly withers, but that it retains its vitality for a long period if the wood only 

 be immersed. Examination of such branches further enables us to determine 

 which part of the woody tissue is more especially concerned in water transport. 

 Although a priori it would appear unlikely that the wood fibres or parenchyma 

 should be the tissues specially concerned in the transport of water, still direct 

 evidence that the vessels are the real agents is not at once attainable. The 

 fact that water does ascend by the vessels, and more especially by their lumina, 

 may be demonstrated (though the mode of proof is scarcely scientifically exact) 

 by placing cuttings in a solution of an appropriate colouring matter (e. g. eosin) 

 and permitting transpiration to take place. From the coloration of the walls 

 of the vascular elements we may deduce the rapid ascent of the solution in 

 the vessels, and all the more readily and clearly if we employ transparent white 

 petals, the vascular network in which appears deeply stained by the pigment 

 after transpiration has gone on for only a short time. Such investigations 

 cannot, however, be carried out on the entire plant owing to the fact that the 

 roots refuse to absorb colouring matters. Further, certain precautions have 

 to be taken in experimenting with cuttings, of which we shall speak later. 

 Obviously, then, the fact that the living wood elements are unable to absorb 

 colouring matters leaves a doubt in our minds as to the validity of such ex- 

 periments ; for although under these conditions a movement of the colour 

 solution is on the whole possible only in vessels which are destitute of proto- 

 plasm, and although its entrance proves more especially that it can ascend 

 in the vessels, still these facts do not prove that in the uninjured plant it uti- 

 lizes the lumina of the vessels exclusively for its ascent. Experiments where 

 the lumina are occluded by the infiltration of foreign substances and thus made 

 impassable for water are, for this reason, much more convincing. Thus Elfving 

 (1882) was the first to place cuttings in cacao-butter, which has a low melting 

 point, and allowed them to transpire whilst standing in that medium, so that 

 the butter rose in the cavities of the vessels. Errera (1886) employed melted 

 gelatine for the same purpose. The infiltrated substances were thereafter 

 coagulated by cooling, and complete occlusion of the lumina of the vessels was 



