266 TRANSFER OF WATER THROUGH THE PLANT. 



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

 sented by more than 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. Detrner observed the greatest outflow 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. 1 



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 satisfactoril}" explained. Baranetzky 

 found that the greatest extravasation of sap from the crown of 

 the root took place in Riciuus between 8 and 10 o'clock A. M., in 

 Helianthus annuus between 12 M. and 2 P. M., and in Helianthus 

 tuberosus between 4 and 6 P.M., the plants being under essen- 

 tially the same conditions. 



716. 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 difficulty occurs in 

 answering the question why the turgescent cortical cells of the 

 root expel their water only inwards into the woody tissues, and 

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

 be helped by the supposition that the micellar structure of the 

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

 and that those facing the exterior of the root are best adapted 

 for permitting filtration tinder high endosmotic pressure." 2 



Among the recorded experiments which show a great root- 

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

 attached to the root of a black birch-tree as follows. The tree 

 stood in moist ground at the foot of a south slope of a ravine, 

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



1 A full and satisfactory treatment of this subject in detail will be found in 

 the following works : 



Schroder : Beitrag zur Kenntniss ilcr 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 (Abhandl. 

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

 experiments cover a wide range. 



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



