178 BOTANICAL GAZETTE Ise?tember 



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the rhizoids of the Marchantiaceae. As we are told by Kamer- 

 ling ( 1897 : 12), after abubble is formed in one of the **pegged 

 rhizoids, it may increase in length downward as well as upward, 

 the water from the lower end flowing past the bubble. Another 

 of Strasburger's experiments (189 1 : 819) furnishes as positive 

 proof as that by direct observation; the same experiments which 

 proved that the Jamin's chain was held less firmly in tracheae 

 than in glass showed that a pressure insufficient to move the 

 bubbles would force water through the same tracheae. 



V. Some of the theories as to the rise of water, and retaining 

 only an historical interest, may be mentioned very briefly. 

 Knight (1801 : 344) ascribed the ascent of sap to contraction by 

 the medullary rays, ''the silver grain,'' which somewhere touch 

 all the vessels. **Their restless temper, after the tree has ceased 

 to live, inclines me to believe that they are not made to be idle 

 wliilst it continues to live.'' If a tangential surface of wood is 

 exposed during the day, the silver grain appear as minute clefts 

 into which pins may be stuck and be found pinched fast at 

 night. This contraction, he thought, might elevate the water- 

 Bischoff (1836, Vol. II, pt. 1:238, 271) believed the sap to 

 be driven up, not in the vessels but in the cells, by the con- 

 tractility of the walls, acting under the influence of the livmg 

 contents. Bohm atone time (1878) held a similar view, omitting, 

 however, as he always did, any dependence on the activity of the 

 protoplasm. He called the movement (p. 230) *' une fonction 

 de I'elasticite des parois cellulaires et de la pression atmos- 

 pherique," But already in the same paper (p. 236) he recog- 

 nized that in cells with rigid walls their elasticity is replaced by 

 that of the air in the lumen, 



Th.Hartig (1853), observing facetiously that his was the age 

 of steam, thought the cavities of the pits might act as millions of 

 little engines so numerous that when warmed even a very little 

 they could drive the water up. 



The agitation of trees by the wind was plausibly used by 

 Spencer to move water in the vessels from the roots, where it 

 w^as most abundant, to the twigs and leaves where evaporation 



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