TURGESCENCE AS A CAUSE OF GROWTH. 565 



laid in water, became very considerably elongated by growth, and at last possessed 

 only 1-97 7o of solid substance. 



The facts here mentioned allow us to explain in the first place why active 

 growth in plants is only possible when the supply of water is abundant. When, in the 

 Spring for instance, the buds of trees, rhizomes, and bulbs unfold, and in the course of 

 a few days numerous shoots cover the gardens, woods, and fields with their leaves, 

 this process consists chiefly in an extension by means of water of the small organs 

 already existing in the winter-buds. Just as a small soap-bubble grows as the air is 

 driven in, and its volume increases a hundredfold or more, so the cells of these 

 organs become distended by the water taken up ; but of course even this comparison 

 falls short of the truth, like others, since the cell-walls distended by water do not 

 become proportionally thinner as they increase in extent, as does the wall of a soap- 

 bubble, but grow with the increase in extent in such a manner that they rather 

 perhaps increase a little in thickness. 



A second fact of fundamental importance for the growth of the cells and tissues 

 is this, that they grow only when turgescent. Turgescence is, if not the only 

 cause, at any rate one of the most important causes of the grov/lh in surface of the cell- 

 walls; or in other words cell- walls only grow in surface — i.e. the organs elongate and 

 become broader — only in proportion as the water contained in the interior of the 

 cells strives to press the cell-walls outwards, or to stretch them. We have already 

 (Lecture XIII) learnt to know this condition of the cells as turgescence, and, 

 to repeat, we understand by this term the hydrostatic pressure which the cell- 

 sap exerts on the cell-wall, so that the extensible but at the same time elastic 

 cell-wall becomes distended. It is obvious that by means of the elasticity of the 

 distended cell-wall a contrary pressure is exerted on the cell-sap which stretches it, or 

 mutual tension occurs; it is this condition which we designate turgescence, and 

 since experience teaches that cells only grow in extent if they are turgescent, we thence 

 conclude that turgescence is one of the most essential causes of growth : the cell-wall 

 grows in the direction of its circumference only so long as it is stretched by the 

 watery cell-sap. The turgescence will cease at once if the growing cell-wall, 

 by the deposition of new particles of substance in it, actually assumes the form 

 which has become forced upon it by distension; but as the cell-wall grows in 

 extent new water is continually being taken up into the cavity of the cell, and thus the 

 already larger wall becomes again distended, and thus the tension due to turgescence 

 is maintained even in the growing cell, until finally another condition comes in where 

 the cell though turgescent grows no more, or where, through the growth of the cell- 

 wall, its passive extension is compensated without further distension being produced 

 by the entrance of fresh water. In these two cases the cell, or the growing organ, 

 is fully grown, i.e. it increases no more in surface, though the thickening of the 

 cell-walls may now begin, and the relations of rigidity especially become altered 

 as described in Lecture XIII. 



It was further stated there that the turgescence of vegetable cells depends upon 

 three things : — (i) By means of the endosmotic action of substances dissolved in the 

 cell-sap, water is continually being absorbed into the cell from without ; this is the 

 moving cause of the whole process ; (2) The water forcibly absorbed into the cell, in 

 spite of the strong pressure which it exerts, must not filter out again, and this is 



