TURGESCENCE AND DROOPING. 567 



These statements of Pfeffer and De Vries harmonise completely with my experi- 

 ments, according to which the strongest turgescence is to be observed in just such 

 parts of plants, the dry weight of which amounts to only a small percentage, which in 

 the main are to be put to the account of the cellulose walls and the protoplasili, so that 

 only a very stnall proportion can come from the substances dissolved in the cell-sap. 



If from our theoretical considerations it results that within a growing cell or a 

 growing mass of tissue such a tension exists between the sap and the cell-wall, we must 

 also suppose that on the cessation of this tension the cell-wall must contract, and the 

 part of the plant concerned diminish in size, and if the extension due to turgescence 

 has taken place chiefly in the longitudinal direction, a corresponding shortening 

 must take place on the cessation of turgescence. There is, however, a very simple 

 means of stopping turgescence: it is simply necessary to prevent the addition of 

 water to growing parts of plants, and to promote the evaporation of water from the 

 cells ; in this case, as already explained, parts of plants which are filled with sap 

 droop and become flaccid, and measurement shows that internodes and roots in this 

 condition are considerably shortened. The cells which become contracted by the 

 evaporation of the water of their cell-sap behave like a soap-bubble, which, hanging 

 from the blow-pipe, contracts and drives back a portion of the inflating air and thus 

 becomes smaller. This shortening on the cessation of turgescence, then, is efiected 

 by the elastic contraction of the cell-walls which had been previously extended by the 

 water which had penetrated by endosmose. At the same time, the diminution of 

 turgescence must cause cells and tissue-bodies to become more extensible, just as 

 a strongly inflated caoutchouc balloon when it loses air again becomes extensible and 

 flaccid (cf. what was said in Lecture XIII). It must also be admitted as a further 

 consequence of our theoretical considerations, that a non-turgescent and drooping part 

 of a plant cannot grow, since otherwise our theory would be false; but it has 

 already been stated above that, as a matter of fact, only turgescent organs grow, and 

 it is easy to convince ourselves by tneasurements that roots and leaves which have 

 become flaccid and droop do not grow. Indeed a marked flagging is not necessary 

 to arrest growth, but only a certain diminution of turgescence : for example, if 

 plants are cultivated at a window, and the soil in the flower-pots kept continually 

 nearly air-dry for weeks and months, then although the leaves remain fresh to a certain 

 extent, the young shoots do not grow ; and something similar is observed in the 

 open in the Spring when the air and the soil are for some time persistently dry. 



We have to thank De Vries for exact and very careful investigations on the 

 significance of turgescence for growth in length. From his ' Untersuchungen uber 

 die mechanischen Ursachen der Zellstreckutig * ' I may quote the following. If growing 

 parts of plants are laid in certain salt-solutions, they lose their turgescence completely 

 in two or three hours, contracting at the same time. Young shoot-axes may 

 thus lose 4 or 5 % of their length. As solutions particularly suitable for this 

 purpose De Vries mentions those of saltpetre and of common salt. The shortening 

 mentioned, accompanied no doubt with a thinning of the part, that is the total diminution 



' Hugo de Vries, ' Unters. iiber die mechanischen Ursachen der Zellstreckung^ Leipzig, 1877, 

 a treatise which must be studied by any one desirous of making himself acquainted in detail with 

 the mechanics of growth. 



