THE MOLECULAR FORCES IN PLANTS. 197 



triquetrum, we find that the entire tissue consists of cells, whose 

 walls are yellowish-brown in colour and strongly thickened. The 

 lamina of the peripheral and of a few central elements (which last 

 suggest a central bundle) are much narro-wer than those of the- 

 other cells. In mosses which have no central strand, or only a- 

 feebly developed one, composed, as is known, of cells much elon* 

 gated longitudinally, practically only the external conduction of 

 water through capillarity has to be taken into account. On the 

 other hand, a well-developed central strand appears to offer the 

 possibility of vigorous internal conduction of water, and experi- 

 ments made with Polytrichum confirm this. A well-developed 

 central strand is in fact present in the stem of Polytrichiim, as we 

 may easily ascertain by microscopic examination of delicate trans- 

 verse sections, and on placing a few stems of Polytrichum formo- 

 sum close together, with their lower ends in water, I found that 

 even the upper leaves of the plant remained fresh. In this condi- 

 tion the Polytrichum leaves stand off from the stem, while in 

 drying they apply themselves closely to it. 



Particular interest also attaches to the manner in which Sphag- 

 nums take up water from the outside and retain it. It will here 

 be sufficient to indicate briefly the structure of the leaves of these 

 plants. We select for microscopic examination Sphagnum acuti- 

 folium, the green, reddish, or intensely red sods of which are easily 

 found. The mature leaf consists of chlorophyll-containing cells,, 

 which are united together to form a network, and of colourless 

 cells, no longer living and containing water or air, which lie 

 between the green cells and have membranes thickened in an 

 annular or spiral manner, and pierced with holes. Through these 

 holes water easily passes into the cells from the outside, and. is 

 retained by them, so that a sod of Sphagnum can take up large 

 quantities of water, acting indeed like a sponge. 1 



1 On the absorption of water by Mosses-, see Oltmanns, Straslurger In* 

 augural-Dissertation, 1884. 



VI. MOVEMENT OF WATER IN PLANTS; 

 75. Root Pressure. 



If it is desired to prove that by osmosis and turgidity forces can 

 be set up in the cells, which are able to drive the cell-sap through 

 the membranes of the cells, and e.g. into the vessels, it will be 



