368 LECTURE XVI. 



years ago, with fresh, vigorous pieces of living Pine. When a film of water was 

 placed with a brush on the upper transverse section of a piece of the stem even 

 I — 3 m. long, it sank in a few seconds into the wood, while an equal quantity of water 

 appeared at the lower section, — a proof that the smallest pressure can be equalised 

 by means of filtration through the wood. By this and other experiments I con- 

 vinced myself, also, that the spring wood of the annual rings is' more permeable by 

 far than the autumnal and heart wood. 



I may take this opportunity also of mentioning the very remarkable fact 

 that the air in the wood-cells, as well as in the vessels of living plants, is in a 

 high degree rarefied — a fact which I had already started hypothetically in 1865, 

 but which is now better known, and can be easily demonstrated. The simplest 

 proof that the air in the wood-cells exerts a feebler pressure than the atmosphere, 

 lies in the fact that a freshly cut piece of wood, when laid in water of like 

 temperature, absorbs it eagerly, as may be easily proved from the increasing 

 weight of the wood. This absorption, however, is simply nothing further than 

 the forcing in of the water by the pressure of the external air, which, in its turn, 

 can only exert this influence when and so long as the tension of the air contained 

 in the wood, together with its aqueous vapour, is feebler than the pressure of the 

 atmosphere; or in other words, if the air-bubbles saturated with vapour in the 

 wood had the same tension as the atmospheric air, it is by no means obvious 

 how fresh wood, the cell-walls of which are saturated with water, and the cavities, 

 at least in part, filled with it, could still take up water; tind this is, of course, still 

 much less the case if the cavities of the wood were already quite full of water. 

 In 1874 Hohnel made known the remarkable fact that when a branch of a living 

 plant is bent down into a bowl of mercury, and there cut through, the mer- 

 cury penetrates far into the vessels. In a few seconds the mercury pene- 

 trates several centimetres into the vessels of both parts of the shoot, and so 

 much the further the wider the vessels are. If the capillary repulsion between 

 the vascular walls and the mercury is also regarded, one is led to the conclusion 

 that the latter must be forced into the vessels by a considerable pressure, or, if 

 one will, suction. The pressing force, however, is no other than the pressure 

 of the atmosphere on the mercury; and this is only in so far effective as the 

 pressure of the air in the vessels is smaller, or, in other yvords, because the air 

 in these is rarefied. The rarefaction may be very considerable, and Hohnel 

 calculated from his experiments that the pressure of the air in the vessels may 

 sink to i or even | of the atmospheric pressure; and I have sought to make 

 it probable that, under certain circumstances, younger vessels and wood-cells may 

 even be completely empty of air and only contain aqueous vapour'- In fact, 



' I have already propounded my view that wood-cells and vessels may be devoid of air under 

 certain circumstances in ' Arbeiten des bot. Inst, in Wzbg.' (II, 1879, p. 324). I may simply add 

 one remark here. When, on the death and disappearance of the protoplasm from wood-cells and 

 vessels, the watery sap contained in their cavities comes into immediate contact with the lignified 

 walls, it is inibibed under the influence of the neighbouring wood cell-walls. The water contained 

 in the recently developed wood-cells and vessels enters into the ascending transpiration current and 

 is carried forward by this. The imbibing force of the wood cell-walls here effective, however, is, 

 as we know already, enormous, and may be able to withdraw the water from the fresh wood-cells 



