PHYSIOLOGICAL 311 



viscid, to the touch because of its embryonic and dividing cells. The 

 white wood is thus exposed, with its youngest "sap-wood" outermost : 

 so called because it is here that the water ascends. The "bark" 

 which slips off for our whistle has its own complexity, with soft 

 bast and its living cells and sieve-tubes, and its hard bast with its 

 tough and flexible fibres (so often important to man), while outside 

 this there is in young shoots a layer of green parenchyma and an 

 epidermis, much as in a leaf. In stems that have to stand the winter 

 a cork layer appears, with uniquely developed characters of im- 

 permeability to water, and of non-conducting defence against both 

 heat and cold — qualities hence again eagerly utilised by man. 



But now let us concentrate on this young wood. It consists of 

 elongated spindle-shaped cells (tracheids) , and also of vessels (tracheae) , 

 formed from the end-to-end fusion of cells ; and both these kinds of 

 elements have thinner spaces in their substantial walls, through 

 which water may pass laterally. The strands of the young wood 

 form the path for the so-called "transpiration current", or, in other 

 words, for the ascent of watery sap; and some investigators main- 

 tain that they also serve for sugary sap passing downwards; and 

 not the soft bast alone, as was formerly believed. If we may compare 

 a plant with an animal (though this is always dangerous, this young 

 wood may be likened to the blood-vessels. Or, dropping the com- 

 parison with animals, the young wood is the main transport system. 

 The fundamental fact is that these wood-strands form a continuous 

 system extending from just below the absorbent root-hair region of 

 every rootlet to the central framework of the leaf. Every cell of the 

 root-hair region and every cell of the central tissue of the leaf is in 

 close contiguity to a wood-strand — to the young wood elements of 

 a fibro-vascular bundle. Thus there is a living and working con- 

 tinuity from the place where soil water is captured to the place 

 where carbon compounds are synthesised. Yet what positive proof 

 is there that the young wood is the ascending pathway for the 

 watery sap ? 



There is a general correlation, suggestive if not convincing, that 

 high-cHmbing plants with narrow stems show strong development 

 of wood-strands, while submerged aquatic plants show a very poor 

 development of this tissue. But more conclusive evidence is afforded 

 by girdling and cutting experiments, which show that removal of 

 the bast is not followed by wilting, whereas cutting the young 

 wood is at once followed by the wilting and probably by the death 

 of the leaves. More precise experiments, utilising some readily 

 recognisable injection material, such as Chinese ink, have proved 

 that the main pathway of the ascending water is by the young 

 wood-vessels or tracheae. The fluid can be seen passing up these 

 microscopic pipes, "swirling into their open ends", as one botanist 

 puts it. The neat converse experiment has been made of plugging 



