576 LECTURE XXXIII. 



De Vries. The shortening of the root, discovered by Filtmann as long ago as 1819^ 

 and then forgotten, and at length again observed by Irmisch and myself, is manifested 

 in the first place by the production of transverse folds on the surface of those parts of 

 the root which have for some time ceased to grow in length; the roots of many 

 marsh-plants, those which grow in water especially, as well as those of Hyacinths, 

 various species of Ins, &c., easily permit the observation. De Vries made marks at 

 determined distances on the primary roots of young plants of Red Clover and 

 Beet, and then allowed them to go on growing in soil or nutritive solution, and 

 found that after 3-6 weeks the regions marked had become shortened by 

 10-15 °/a ^'^^ ^^ some cases even 20-25 % ^^ *-^^^^ length. On letting 

 the main roots of actively vegetating plants of the Caraway, Teazle, and 

 Artichoke, taken out of the soil and separated from the tuft of leaves, lie for 

 3-4 days in water, they shortened about 4-8 7o, increasing in thickness the while 

 (sometimes even 4-8 7o)) and accordingly an increase in volume and simultaneous 

 stiffening took place. Isolated parts of the tissue in water showed the same 

 changes : the axial strand as well as the parenchymatous cortex became shortened in 

 the longitudinal direction and extended transversely. According to De Vries it is 

 in both the parenchymatous tissue alone which initiates this change, which in fact was 

 directly measured microscopically by him : the cork envelope as well as the vessels and 

 bast-like fibres become passively bent, and cause in part the formation of the transverse 

 folds referred to. De Vries sums up the most important points of his observations as 

 follows : — ' The contraction (shortening) on taking up water is a phenomenon of 

 turgescence, and is reversed by all agents which stop turgescence. This is shown most 

 simply by the fact that the contractUe roots do not shorten on flagging, as growing 

 root-apices or stems tend to do, but become elongated. In the same way they lengthen 

 if the protoplasm is in any way killed, or if it is forced to separate itself from the 

 cell-wall by the action of salt-solutions. On the cessation of turgescence the tissues 

 become drawn together in the transverse direction.' 



The shortening effected by the absorption of water is rendered permanent by 

 subsequent growth, just as the turgescent elongation of stems and leaves becomes 

 fixed by subsequent growth. It at once results from this, as De Vries remarked, 

 that just those roots which have the best marked transverse folds exhibit the most 

 distinct shortening in water. Thus as we explain the longitudinal extension by 

 turgescence of parenchyma-cells in shoots, chiefly by the fact that the cellulose 

 walls are more extensible in the direction of their length than transversely, we must, 

 on the contrary, assume as regards the root-parenchyma that the cellulose walls 

 are more extensible transversely than longitudinally, since it is always to be main- 

 tained that the hydrostatic pressure which causes the turgescence is equal in 

 magnitude on all sides. 



It may be concluded, from a long series of various phenomena in the life of 

 plants, that the form and size of the individual cells within the tissue depend to 

 an important extent upon how they are connected with neighbouring cells. In other 

 words, we may assume that the shape and size of each tissue-cell is determined 



' Hugo de Vries, ' Über Verkürzung pflanzlicher Zellen durch Aufnahme von IJasscr," Bot. 

 Zeit., 1879, p. 650. 



