January i i, 190^ 



NA TURE 



247 



the question ; and while some of the difficulties which 

 present themselves upon detailed examination may be over- 

 come, others are less easily surmounted. In the trunk 

 of a tree it is only the cambium itself which can properly 

 be regarded as consisting of living cells — if we use the 

 expression in its usual sense. Besides protoplasm the 

 cambium cell contains a nucleus, and splitting up to form 

 the growing wood is evidently and unquestionably a living 

 cell. The sap cannot, however, be considered as entirely 

 rising through the cambium alone; while the medullar} 

 rays and wood-parenchyma, although they both contain 

 protoplasm or such like organic substances, are in no 

 other respects like living tissue. They may be regarded 

 simply as store houses containing nutritive matter, or as 

 actively engaged in the plant's circulation, or as acting 

 in both capacities ; but scarcely can they be described as 

 centres of vitality. 



The theory which most readily commends itself to our 

 ideas of probability is that which regards osmosis as the 

 primary and all potent cause of the sap's ascent in plants ; 

 nut taking place in the root alone, as was supposed by 

 those who advocated the earlier theory, but active through- 

 out the whole height of every tree. Concerning osmosis 

 itself it is well to remember that the phenomenon con- 

 tains nothing transcendental or beyond the reach of ordinary 

 molecular physics for its complete explanation. We know 

 that the particles of a liquid, though far from possessing 

 that mobility which in a gas is due to the great extent 

 of free path enjoyed by its molecules, are constantly in a 

 state of translational motion with regard to each other. 

 The phenomenon of diffusion in inorganic solutions 

 obviously suggests the conclusion that this capacity de- 

 pends largely upon the relative simplicity which charac- 

 terises the molecule's construction. Especially is this idea 

 forced upon us when considering the relative diffusibility 

 of various solutions. Here we find the rate of diffusion 

 bearing a definite relation to the solution density ; the 

 square of the time of equal diffusion being in the case of 

 such solutions equal to their solution density. It would 

 appear, apart from all questions of chemical combination, 

 that each molecule — or perhaps group of molecules — of the 

 solvent becomes attached to a certain number of molecules 

 of the dissolved substance ; this complex group holding 

 together so far as diffusion and osmosis are concerned. 

 Under such circumstances it is only to be expected that 

 the rate of diffusion, which means the passage through 

 intermolecular fluid spaces, or of osmosis, which consists 

 in the passage through interstitial spaces in a porous 

 solid, should vary quite as much as is observed to be the 

 case. 



It is true the relatively small differences in the rate of 

 passage through porous bodies observable when inorganic 

 solutions are compared with each other, seem t<> depend 

 principally upon chemical action between the solutions and 

 the substance^ but in the much more marked difference 

 observed when we compare the dialysis of crystalloid and 

 colloid bodies, this does not depend on anv such action. 



Xow in a plant we have a system in which the process 

 of nutrition is going on at both ends, the roots and the 

 leaves of a tree ; it is, however, the organic colloid sub- 

 stances which arc manufactured in the leaves, while in 

 most part inorganic or crystalloid compounds are absorbed 

 by the roots. Both forms of nutriment are required at 

 every point of the tree. The colloids have to descend, the 

 crystalloids and water have to be raised. Constant 

 evaporation from the leaves by maintaining " negative " 

 pressure in the vessels greatly facilitates the rise of water 

 from below; but the motive power is osmotic action taking 

 place between any or every pair of cells in the chain. 

 The "air bubbles" which form a chapelet de Jamin in 

 the vessels and prevent the fall of any water previously 

 raised by osmosis, at times when demand falls or moist 

 air saturating the leaves supplies all water necessary from 

 above,' very possibly convert the fluid column itself into 

 the equivalent of a porous body suitable for the action of 

 osmosis to make itself felt between each pair of drops as 

 they hang suspended in the vessels. The film of liquid 

 surrounding each bubble is as a narrow space in a porous 

 body through which the simpler and smaller groups of 

 atoms can more readily pass than can that cumbrous 

 collection which constitutes the physical molecule of any 



NO. 1889. VOL - 7ol 



colloid solution. This action adapts itself exactly to the 

 plant's requirements. Should anv sap fall short at any 

 point of water or crystalloid solution, osmotic action 

 immediately supplies what is required from below ; while 

 the enormous pressures which dialysis can bring into play 

 leave gravity intc quantite ncgUgeublc. The details 

 characterising this action as observable in conifers, as 

 distinguished from dicotyledonous trees, will doubtless vary. 

 The former suggest to most unbotanical minds the idea 

 of an earlier "and less highly developed type. The 

 tracheides of a conifer act simultaneously as conducting 

 vessels and as hollow cells in the structural framework 

 of the trunk considered as a beam; while these purposes 

 are more or less differentiated in the case of dicotyledonous 

 plants. In this latter case the wood-parenchyma cells 

 surrounding the " vessels " — filled as they are with colloid 

 matter — appear to supplement the action of the medullary 

 ravs, not here in such close connection' with conducting 

 tracheides as they are in the case of conifers. The 

 presence of all this colloid matter, scattered throughout 

 the conducting mass and closely connected with the 

 tracheides or with the fitted vessels, probably act as a 

 reservoir and enlarge the sphere of the osmotic action, thus 

 avoiding violent changes and preventing any very noticeable 

 difference in sap density occurring throughout the tree's 

 height. A gradient, however, marking difference in pro- 

 portion between colloids and crystalloids must necessarilj 

 exist whenever water is rising, and this would naturally 

 be expected to follow the introduction of these different 

 forms of nutritive matter at opposite ends of a chain. 



ELEMENTARY GEOLOGY. 1 



IF a new elementary text-book of geology is really 

 in request, no better author could be found than 

 the president of the Geological Society of London for 

 1905. We venture to prefer this work to his some- 

 what similar " Agricultural Geology," and hope that 

 candidates for a diploma in agriculture will now make 

 use of both. The author, while engaged upon his 

 task, appears to be absolutely devoid of the emotion 

 which " nature-studv " provokes in other men in 

 various measure, and his introduction, if a little cold, 

 should lead to accurate observation and understand- 

 ing. The photographic illustrations are refreshingly 

 large, and include successfully the forms of familiar 

 fossils and even of flint implements. Four pins fixed 

 in a dull white wall would, however, have served as a 

 more satisfactory support for a helpless belemnite than 

 the operating table and other apparatus displayed in 

 Fig. 29. The striking relic of a Triassic land-surface, 

 photographed by Prof. H. E. Armstrong (Fig. 27), 

 is here reproduced, as an example of the admirable 

 landscapes in this volume. 



Our onlv question about the book is as to the 

 class for 'whom it is intended. In the frequent 

 absence of systematic scientific training in English 

 schools— things are fortunately different now in 

 Ireland scholars may come up to our universities 

 completely ignorant of chemistry and physics. 

 The\ may also be ignorant of the animal and 

 vegetable forms around them, and they are certain 

 to believe that coral is a substance laboriously 

 manufactured by an insect. We take it that, on con- 

 tact with Dr. Marr and the well-known Woodwardian 

 collections, such scholars may become attracted 

 towards geology. Hence, in the present work, this 

 complex subject, relying for its evidence on almost 

 every other science, is treated as one to be laid before 

 babes, who have never handled a blowpipe, or stroked 

 tie back of a cat to see that it possessed a spinal 

 column. From these pages the reader may " proceed 

 to the perusal of more advanced treatises." But what 



1 "An Introduction to Geoloey ." By J. E. Marr. ScD.. F.R.S. 

 Pp. viii+230. (Cambridge; University Press, 1905.) Price31.net. 



