200 JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 



it from the soil. The cells of the root somehow force it up the wood 

 conduits to such an extent that it may balance the pressure of several 

 atmospheres. Now, of course, it is very convenient that such an 

 arrangement should exist, for it enables the water to be pumped up 

 the trunks of trees in the spring. But that is no explanation whatever 

 of its occurrence, nor indeed are we able as yet to give a completely 

 intelligible physical explanation of how it comes about. Various so- 

 called explanations of course have been suggested, but none has been 

 able to stand the test of physical and mathematical investigation. 

 But if we cannot solve the whole problem, we can follow up some 

 parts of it, and as this question of water supply is of paramount 

 interest to all who cultivate plants, I may perhaps be forgiven for 

 dwelling more at length upon it, especially as much misapprehension 

 exists on the subject. I may assume that the ordinary structure of a 

 typical root is understood, and the annexed figure (Fig. 43) will serve 

 to recall its appearance in transverse section. 



What occurs is that the root hairs absorb the water from the soil, 

 and this water passes through the intervening cells till it reaches 

 those which abut directly on the wood. From them it is forced into 

 the conduits of the wood as already stated. How this wonderful thing 

 is brought about we do not really know, nor is it easy directly to 

 investigate it. We think we understand pretty well how the water 

 is extracted from the soil ; what we do not know is how it is continually 

 forced out at the other end. To put it in more exact words, we do 

 not know for certain the source of the energy which maintains the 

 forcible flow, nor do we know at all how the energy itself is utilized in 

 the process — a very unsatisfactory state of things, but a knowledge 

 of exactly what it is that we wish to ascertain is the first step in any 

 investigation. I hope, at any rate, I have made clear the difference 

 between understanding a phenomenon and the mere recognition of 

 its utility. 



But there is one fact in root structure which I think is worth 

 noting in connexion with root pressure, and this relates to the structure 

 of the endodermis. The endodermis forms a continuous sheet of 

 jacket cells which completely cuts off all direct communication between 

 the vascular cylinder on its inside and the rind with its air spaces on 

 the outside. Certain ceils of the endodermis are definitely the tracks 

 through which the water passes inwards, the others are often strength- 

 ened in various ways. The evidence appears irresistible, on closer 

 examination, that the presence of this layer in the roots — it is always 

 present in roots, whilst in the stem it may or may not be recognizable— 

 is intimately bound up with the process of water movement. At any 

 rate, not to go more deeply into the matter, it constitutes a beautifully 

 constructed arrangement for preventing water that has once passed 

 through it from leaking outwards except by passing through the interior 

 of the cells themselves — and thus it is pretty certainly intimately 

 connected at any rate with the maintenance of root pressure. 



Now I have said that the physical basis of life, the protoplasm, 



