SLOWER CURRENTS OF WATER. 233 



roots, Stems, and leaf-stalks of the land-plants thus serve for the rapid movements of 

 water, caused by transpiration ' ; this does not preclude, however, that slow move- 

 ments of water of the most various kind may also take place in all the other 

 forms of tissue of these plants. In the first place, it is quite certain that the water 

 in the younger absorbing roots must pass through the external parenchymatous 

 layers of tissue, in order to reach the axial woody bundles ; and it is easily 

 intelligible, again, that the thin fibro-vascular bundles of the petiole, which spread 

 out and branch within the lamina as the venation, transfer their water to the 

 parenchymatous chlorophyll-cells of the leaf, to which end, in fact, the whole 

 mechanism of the water currents is set in motion. In the same way the cortical 

 parenchyma of the shoot-axes, and even their epidermis, require certain quantities of 

 water in order to make good the feeble evaporation at their surfaces ; and there is 

 no doubt that they draw this laterally to themselves from the stream ascending in 

 the wood. Finally, the drooping and renewed turgescence of shoot-axes and thick 

 leaf-stalks abounding in parenchyma, teach us that the water ascending in the 

 woody bundles also passes over into the parenchymatous layers, because otherwise 

 the restoration of the turgescent condition would not be possible. The disdnguishing 

 feature about all these movements of water in the non-lignified tissues, however, is 

 the slowness and difficulty with which they occur ; and this is clearly indicated in 

 the whole organisation of the land-plants. The course of the woody bundles conducting 

 the water in the leaves is so regulated, and the ramifications of the venation so con- 

 sdtuted, that the water conveyed in the latter need only traverse an exceedingly 

 short distance in order to enter into all the parenchyma cells of the leaf. We see also, 

 at the same time, why the venation, especially of the strongly transpiring thin leaves 

 of Dicotyledons, divides up the entire area of the leaf into so many exceedingly small 

 areolae ; because by this means the paths of the water particles in the badly con- 

 ducting parenchyma are rendered the shortest possible. In like manner the path 

 taken by the water from the woody portions of the stem across into the cortex is 

 never more than a few millimetres long, although a rapid supply is not at all necessary 

 here. In the cases where large gourds, apples and pears, tuberous roots and tubers 

 gradually absorb large masses of water into their parenchymatous tissue, which is not 

 traversed by strong woody bundles, it simply depends not upon a rapid transpira- 

 tion-current, but, on the contrary, upon an exceedingly slow movement of water, just 

 sufficiendy copious to bring about growth — the increase in volume of the cells. In 

 contrast to this we find the woody body, or the strands of tissue equivalent to it, always 

 the more strongly developed the more it is a matter of conveying large quantities of 

 water in a short dme to widely distant and large transpiring surfaces; and, remarkably 

 enough, these considerations also confirm the fact that the same distribution of the 

 lignified layers and strands, by which the elasticity and rigidity are obtained in the 



' That the wood cell-walls differ essentially and entirely from other cell-walls as regards their 

 conductibility for water, and the minute quantities of substances dissolved in it, and that, while 

 imbibing less, they hold the water in a very mobile condition, I have also made evident in the 

 treatise mentioned in note i, p. 230. This is opposed to the assumption previously current, and 

 especially contrasts with the views put forth by Nägeli and Schwendcner. In note i, p. 20S, all 

 that is necessary has been mentioned concerning the fact that imbibition is essentially different 

 from capillaiily, although the two have hitherto been always coufoumlcd. 



