548 



Wittrock^ percc*i\e(l in tlie phenomt-na of mo\c'mcnt a protection 

 against the cold of winter. For example, the evergreen root-leaves of 

 numerous plants hend backward and downward so that at last the outermost 

 part of the under leaf surface seems i)ressed against the soil ; in summer 

 they have a slanting position. This is especially clearly noticeable in Hypo- 

 choeris maciilaia L.. Gcum urbanum L., Ccrcfolium satii'ion L., and others. 

 Also a few early spring plants like Ranunculus Ficaria L. l)eha\e similarly. 

 Hartig recognized in these phenomena rather a zciltiiuj of the f^arts of the 

 plant, resulting from the limpness of the cells, from which the water has 

 been frozen out into tlic intercellular spaces. Since the freezing of tlie 

 water in the \arious {jarts of the organ will dilTer according to the age and 

 maturity of the tissue, the difference in the movement, due to frost, might 

 be explained in this way. 



."^^uch phenomena of movement, ho\\e\er, seem in no way connected 

 with the formation of ice and are onl}- extreme cases of thermonastic reac- 

 tion which, as Pfeffer- states, are exi)ressed in the nocturnal droo|)ing of 

 the blossoms, leaves and shoots. X'ochting' obser\ed in M'nuuhis 'filinfiii 

 Rgl. that shoots of a certain age grow upward in tlie spring with a high 

 temperature or maintain a horizontal direction with a low one, while in 

 case they ha\ e developed an upright position, they reassume the horizontal 

 one. Light and humidity have no influence. He believes that with con- 

 tinued low temperatures the jilant might develop only creeping shoots on 

 which blossoms are never [)roduced. This sensitiveness ceases in the blos- 

 soms which arc termed psycho-clinic. Lidforss' concludes from nunK•rou^ 

 observations on Holosteum, Lamium, \''eronica, etc., with which klinostatic 

 experiments were also made, that in such moxements not only changes in 

 turgor are concerned but actually the effects of stimulation. With a higher 

 temperature the petioles are negatively geotropic, but in temperatures below 

 6 degrees, they are dia-geotropic and epinastic. Here, howe\er. the light 

 acts as a modifier since, with its exclusion, the petioles, in spite of the lower 

 temperature, are no longer dia-geotro])ic but negatixely geotropic. 



The moxements of the peduncles of .Incmonc ncniorosa are, on tlie 

 other hand, of a purely thermonastic nature. The}- curl downward with a 

 lower temperature but stand upright with a higher one. 



In petioles and leaf surfaces, the resumption of a horizontal position is 

 often noticed, or a bending backward below the horizontal plane on taller, 

 upright axes. In this, however, we wish to emphasize the fact that the 

 movements take place usually in the points and are not alwa}s of the same 

 nature in the same plant. It can hai)i)en that, in compound leaves, some of 

 the leaflets turn upward while the majority bend downward; that, therefore. 



1 Bot. Ges. zu Stockholm. Sitz. v. 24. Oktob. 1883; cit. Bot. Centralis. 18; 

 No. BO. p. 350. 



2 Pfeffer. PfianzcnphysioloRie, 2d edition, v. II (1904), p. 495. 

 •T Bot. .Tahresb. 1898, I, p. 582. 



4 I^idforss, Bengt. tJber den Geotropi.smu.s einig-er Fruhjahrsi)ttanzen. .Jahrlj. 

 wiss. Bot., V. 38, 1902, p. 343. (Z. f. Pflanzenkrankh., 1903, p. 277.) 



