243 LECTURE XIV. 



themselves. We have thus to imagine the water of imbibition of the latter as being in 

 motion; and this is the main result of our considerations hitherto'. Any one may 

 easily supply a striking proof of this, moreover. If the delicate stem of the Hop, 

 Flax, etc. be sharply bent beneath a few transpiring leaves, the cells at the sharply 

 bent place must all be so compressed together that cavities containing water no 

 longer exist. Thence follows, however, that the water ascending into the transpiring 

 leaves can only go through the walls of the wood-cells ; and that this actually 

 occurs, follows from the fact that the shoots remain fresh above the sharply bent 

 place — they remain thus fresh for weeks and months, even in the sunshine in 

 the open. 



The great mobility of the imbibed water of the wood cell-walls already estab- 

 lished above, and on which the whole phenomenon of the water-flow thus depends, is 

 a specific property of the lignified cell-walls ; a fact which appears the more striking 

 when it is known that other cell-walls capable of swelling to a great extent, though 

 it is true they absorb water in large quantities, hold it immovably fast. This is very 

 conspicuous, for example, in the case of the stalks of Laminaria, the main mass of which 

 consist of cell-walls capable of swelling; if a fresh stalk of this Alga is so placed in water 

 that some centimetres project above, the immersed part remains swollen, but the part 

 projecting above dries and shrivels up — a proof that the water ascends in it not at all 

 or only exceedingly slowly. Ordinary parenchyma cells appear to behave similarly. 



As further characteristic of the wood cell-walls, in so far as they are the con- 

 ducting organs for the ascending water, it is a remarkable but easily confirmed fact 

 that they lose for ever their main property of conveying water when they have once 

 become air dry. It would be utterly in vain to look for an ascent of water at all 

 rapid in an old dried up rod of wood placed in water. The wood once dried has, 

 it is irue, the capacity of still becoming saturated with water of imbibition ; but the 

 mobility of the latter no longer exists. 



It is evident that by the drying of the wood cell-walls, an essential alteration of 

 their molecular structure has taken place, which, however, is not to be detected 

 with the microscope. Nevertheless this fact is established, and it is of the greatest 

 importance for the life of plants. The so-called freezing of woody plants in the 

 long- continued cold of winter may serve as proof of this; for this phenomenon is 

 something quite other than the freezing of succulent shoots and living leaves in the 

 late autumn or in the spring time. The dying of trees in long-continued winter cold 

 depends to a great extent on the drying up of their wood, as we may convince 



' For the guidance of readers not sufficiently acquainted with phytotomy, it may not be super- 

 fluous to quote the following from my treatise, ' ffber die Porositdt des Jffohes,^ p. 392. 'The wood 

 consists of a framework of lignified lamellae, which enclose cavities (cell-cavities). According to 

 circumstances the cavities may contain water, or air (with aqueous vapour), or both. The walls 

 themselves may be dry, or contain water (imbibed) ; and their volume or condition of swelling alters 

 with the amount of water contained. The cell cavities of the wood are capillary spaces ; the cell-walls, 

 on the contrary, contain no capillaries into which liquid or air could directly penetrate. In order to be 

 able to judge of the movement of water in the wood produced by transpiration and other causes, it is 

 necessary to sharply distinguish between the capillarity of the cavities and the imbibition of the cell- 

 walls.' That the ascending transpiration-current moves in the substance of the wood cell-walls 

 I assumed (though with a certain reserve) in my ' Pflanzen-physiologie' ^. 216: it was first expressed 

 definitely, however, in my ' Text-book.' 



