228 Practical Plant Biology. 



surface of the water adhering to these structures receives all 

 the support of the thick parts of the wall when a rupture occurs, 

 while, as long as none develops, the resistance to the flow is 

 that of the thin pit membrane. 



The recognition that the water rising in the vessels and tra-cheids 

 is usually in a state of tension shows why it is advantageous 

 that the supports in these tubes should be of the form of in- 

 ternal hoops and spirals. These are forms of support pre- 

 eminently suited to resist crushing forces such as the tensile ad- 

 hering water would exert. It is interesting to note that while 

 the strength of the tubular woody walls and their internal supports 

 are utilised to counteract the tendency of the vessels and tracheids 

 to collapse, it is the osmotic pressure exerted by the solutes which 

 prevents the thin-walled mesophyll cells from crumpling in under 

 the tension. 



The evaporation, kept up from the leaf-cells by the inflow of 

 heat there, is by far the most important factor in raising water in 

 the plant ; but at certain periods, viz. early spring, when the buds 

 are opening, and before the leaf-cells can evaporate, and sometimes 

 in warm nights when the moisture of the air checks evaporation, an 

 upward current of water may be detected in the wood. This 

 appears to be due either to the osmotic pressure of the solutes in 

 the wood of the root drawing in water and raising it in the stem, 

 or else to a pumping action located in the outer cells of the root. 

 The action is spasmodic and temporary. The force exerted is 

 called root-pressure. Probably its chief importance is to force 

 water into those tubes where ruptures have occurred and so re- 

 establish the columns of water which will transmit the tension to 

 raise the transpiration current. 



PRACTICAL WORK. 



Make a sketch of a plant of Ranunculus bulbosus or of R. repens, 

 showing the leaves, stem and roots. 



Cut across the roots and immerse their cut surfaces in a watery solution 

 of eosin. While transpiration is drawing up the solution into this plant, 

 prepare a number of thin transverse sections of a root, a stem and a leaf. As 

 they are cut transfer them into water in a staining glass so that they may 

 be ready for examination. 



In the transverse section of the root note the outermost layer forming the 

 surface. It is the pilifcrous layer and its cells here and there grow out to 

 form root-hairs. Within this is the thick cortex In transverse section the 

 cells appear rounded and are thin walled. Their cavities are chiefly occupied 

 by starch grains. Between thtm small intercellular spaces are seen. The 

 cortex is bounded internally by the endodermis. Its cells exhibit an oblong 

 outline. Their walls are thickened and somewhat cutinised and so more or 



