266 TRANSFER OF WATER THROUGH THE PLANT. 



sustain a column of water 18.47 feet in lieiglit, a change repre- 

 sented by more tliau 44 feet of water." 



714. The pressure of the sap rises and falls with the tempera- 

 ture. The greatest pressure in ligneous plants is found when a 

 cold night is followed by a warm morning. This has been ex- 

 plained by the expansion of the air contained in the wood-cells 

 and ducts. Detmer observed the greatest outtlow of sap in the 

 case of the herbaceous plants Begonia and Cucurbita to be at a 

 temperature of from 25° to 27" C, and that the outflow ceased 

 at 32° for Begonia, at 43° for Cucurbita.' * 



715. Besides the variations both in bleeding and in pressure 

 of sap due to external influences there are some periodical 

 changes which are not yet satisfactorilj- explained. Baranetzk}' 

 found that the greatest extravasation of sap from the crown of 

 the root took place in Ricinus between 8 and 10 o'clock a. m., in 

 Helianthus annuus between 12 m. and 2 i>. m., and in Helianthus 

 tuberosus between 4 and 6 p.m., tiie plants being under essen- 

 tially the same conditions. 



71G. The great pressure exerted by sap under certain condi- 

 tions is thus explained by Sachs. From the root-hairs, into 

 ■which the water comes by osmosis, it passes by osmosis into the 

 parenchymatous cells of the cortex. " But a difficult^' occurs in 

 answering the question why the tnrgescent cortical cells of the 

 root expel their water onlj- inwards into the woodj' tissues, and 

 not also through their outer walls. We may, however, here 

 be helped by the supposition that the micellai- structure of the 

 cell-walls is different on the outer and inner sides of the cells, 

 and that those fac^ing the exterior of the root are best adapted 

 for permitting filtration under higii endosmotio pressure." " 



Among the recorded experiments which sliow a great root- 

 pressure is one by Clark, described by him thus : " A gauge was 

 attached to tlie root of a black bircii-tree as follows. The tree 

 stood in moist ground at the foot of a south slo[)e of a ravine, 

 in such a situation that the earth around it was shaded by the 



1 A full ami satisfactory treatme.nt of tliis subject in detail will he found in 

 the following works : — 



Schroder : Beitrag zur Kenntniss der Friihjahrsperiode des Ahorn (Pringsh. 

 Jahrb., vii., 1869). In this, the spring phenomena of the maple are clearly- 

 given. 



Baranetzky : Untersuchungen iiber die Periodicitat des Blutens (Ahhandl. 

 des naturforschende Graellschaft zu Halle, 1873). In this memoir the 

 experiments cover a wide range. 



2 Text-book of Botany, 2il English edition, 1882, p. 688. 



