NOTES AND MEMORANDA. 149 



2nd. The cells whicli directly transpire — those of the epidermis — 

 are generally destitute of chlorophyll ; they do not assimilate, and 

 cannot produce matters capable of causing an osmotic diffusion. It is 

 probable that they contain nothing but water, which cannot be con- 

 centrated by evaporation. 



3rd. If the evaporated water is replaced by the action of osmose, 

 the leaves of the plant which assimilate in moist air would be covered 

 with the water which is given off, and the intercellular spaces would 

 also become filled with water. This has, however, never been ob- 

 served. 



dth. In a green plant exposed in a damp and dark place, the dif- 

 ferences of osmotic tension in the cells of the leaves would gradually 

 be effaced by the consumi)tion of the osmotic substances, or by their 

 departure into the stem. The leaves remain fresh when the plant 

 is transported into dry air without permitting access of light. 



5th. If the movement of water in the leaves were produced by the 

 differences in density of the contents of the cells, it would act in the 

 same manner in parenchymatous wood, a supposition which will not be 

 maintained. 



If the movement of water in the leaves is not due to osmose, this 

 is still more the case with wood, the cells of which in general only 

 contain air when transpiration is very active. Some authorities be- 

 lieved till quite recently that the force of absorption by the roots may 

 have the power of forcing up water even to the topmost boughs of 

 trees. But a considerable number of facts are in opposition to this 

 theory. In a great number of cases it is imj)ossible to prove the 

 existence of any such vis a tergo. 



M. Boehm, in conclusion, says that, in the parenchymatous tissues 

 filled with sap, the movement of water excited by transpiration is a 

 function of the elasticity of the cell-walls and of the atmospheric 

 IH-essure, and in cells with rigid walls the elasticity of the wall is 

 replaced by that of the air enclosed in the cells. The presence of a 

 certain quantity of air in the cells of the wood which conduct the sap, 

 far from being a hindi-ance to the ascent of the sap, is on the con- 

 trary an indispensable agent in the production of this movement. 



Some have maintained that the ligneous cells of plants in full 

 transpiration contain nothing but air, and that, as no water is seen in 

 them, therefore there is none. But the author pointed out, as much 

 as fifteen years ago, when all micrograph ers believed the contrary, 

 that the fibres of Coniferte are closed, and not open. The position 

 of the membrane of the bordered pits eviJently depends on the 

 differences of tension in the two adjoining cells, and corresponds to 

 the direction of the current of the sap. The constant presence of a 

 certain quantity of water in the ligneous conducting cells is an un- 

 doubted fact. 



The movement of the tvater excited in plants hy transpiration is a 

 phenomenon of filtration dependent on the differences of pressure in 

 adjoining cells. 



Growth of the Root of Phanerogams. — M. Ch. Flahault has 

 made an elaborate anatomical study of the structure of the apex of 



