KATE OF ASCENT IN THE STEM. 261 



fresh surface in contact with the solution, when after a short 

 time the dye will mount through the flower-stalk and tinge the 

 parts of the perianth according to the course of the bundles. 



699. Rate of ascent. The following are some of the discor- 

 dant results obtained by the methods mentioned in 694 : 



Name of plant. Bate of ascent per hour. Observer. 



Prunus Laurocerasus . . 42-100 cm McNab. 



Salix fragilis 85 " Sachs. 



Vitis vinifera 98 " " 



Nicotiana Tabacum ... 118 " " 



Helianthus 2200 " Pfitzer. 



700. But little is known as to the reason of the high conduct- 

 ing power of ligneous tissues. That it is not wholly due to 

 capillarity (as has been suggested on account of the abundance 

 of ducts of small calibre in most wood) , is shown by the struc- 

 ture of the wood of coniferous plants in which no ducts are 

 present. Again, at the very time when the evaporation from 

 leaves of plants is most rapid, and the transfer of water to sup- 

 ply the loss must be greatest, the cavities of the ducts are not 

 wholly filled with liquid, but contain a considerable amount of 

 air ; whereas according to the theory of capillarity they should 

 contain only liquid. By a very ingenious series of experiments 

 Sachs has determined the relative amount of space occupied by 

 the cell-walls, water, and cavities in several fresh woods. In 

 the case of fresh coniferous wood he found the following ratios 

 in 100 cubic centimeters of wood: 



Cell- wall, reckoned as dry 24.81 



Water, in the cell- wall and in the cavities 58.63 



Air-spaces 16.56 



But, as Sachs says, since neither intercellular spaces nor ducts 

 are present in this wood, the 16.56 per cent of air must be con- 

 tained in the cavities of the wood-cells ; and further, since the 

 cell-walls can take up only about half their volume of water 

 (say 12.4 cubic centimeters), the remainder (46.23 c.c.) must 

 exist in the cell-cavities. 



701. The method of determining the amount of water held by 

 the cell- walls of dry wood is the following : 



A thin cross-section of fresh wood is hung up in dry air until 

 it ceases to lose weight. During drying a crack appears, run- 

 ning from the centre to the circumference. After ascertaining- 

 the weight of the disc thoroughly dried (at 100 C.), the wood 

 is suspended in a saturated atmosphere until enough water is 



