ROOT-PRESSURE. 201 



shorter scales and rather thicker styles. Since the male occurs in 

 Britain, it is probable that the female may also be found. 



Andersson, who says of S. viridis, that it is almost never seen 

 except either as a cultivated plant, or as growing in the company of 

 its parents near cultivated ground, distinguishes three chief modifi- 



cations, namely—/ 



viridis, the most 



intermediate form ; and albescens, approaching S. alba. All three 

 occur, I think, in Britain. 



The synonymy of S. fragilis and S. viridis, as regards some of 

 our British books, is as follows : — 



Salix fragilis L. — S. Russelliana Sm. E. B. t. 1808; Hooker, 

 Br. Flora, 4th ed. 358, 14. S. fra#M$ t var. f3. Hooker & Arnott, 

 Br. Flora, 8th ed. 401, 10. S. fragilis, var. a. gennina, Boswell 

 Syme, E. B. 3rd ed. viii. 20G (exclude the plate)/ S. riridis, Bos- 

 well Syme (not Fr.), E. B. 3rd ed. viii. t. mcccviii. (exclude the 

 description). S. fragilis, var. S. Russelliana (Sm.), Babington 

 'Manual,' 8th ed.324, 3. 



Salix viridis Fr., And. — S. fragilis, Sm., E. B. t. 1807 ; 

 Hooker, Br. Flora, 4th ed. 358, 13 ; Boswell Syme, E. B., 3rd ed. 

 viii. t. mcccvi. (exclude description). S. fragilis, var. a. Hooker & 

 Arnott, Br. Flora, 8th ed. 401, 10. S. viridis, Boswell Syme,E. B. 

 3rd ed. viii. 207 (exclude the plate). S. fragilis, var. f3. S. fragilis 



(L.), Babington ■ Manual,' 8th ed. 324, 3. 



ROOT-PRESSUKE.* 



By C. B. Clarke, M.A., F.R.S. 



I first read some extracts from ' Sachs' Text-book,' pp. 600— 

 613, to remind the meeting what the accepted doctrine regarding 



root-pressure is : 



" Another kind of motion of water in the plant, depending not 

 on suction but on pressure from below, is caused by the roots. It 

 is the root-pressure which forces out drops at particular points of 

 the leaves." 



"The width of the capillary tubes is much too great to raise 

 water to a height of 100 feet or more." 



" The question whether the attraction of the cell-walls for 

 water is sufficiently powerful to sustain the weight of a column of 

 water of the height of 100 or even 800 feet may be answered in 

 the affirmative. 



" Eoot-pressure has no share in the ascent of the water, at the 

 time when transpiration is active." 



" With the exception of times when the transpiration is small, or 

 when drops exude from the leaves, no root-pressure exists when the 

 plant is uninjured." 



Wat 



in water." 



Spoken at the meeting of the Linnean Society, May 3rd. 



