ROOTS AS EXCRETING ORGANS 61 



regulate their transpiration within certain limits. They 

 do so by various means, the chief of which are the water- 

 proofing of the outer walls of the cells of the epidermis, 

 and the closing of their stomata by the guard cells. 



Roots as Excreting Organs. — From the foregoing, 

 one will correctly infer that roots not only absorb but 

 that they also get rid of whatever disssolved substances 

 they may hold in excess, as compared with the soil solu- 

 tion, provided the cell membranes are permeable to 

 these substances. The roots discharge carbon dioxid, 

 certain organic acids, and various other substances. 

 Some of these affect the solubility of some of the soil 

 constituents, other excreta are poisonous and if allowed 

 to accumulate by neglect of suitable alternation of 

 crops, will impair the productiveness of the soil. 



Secretion and Excretion. — The same physical prin- 

 ciples apply to the secretion of sugars, the excretion of 

 waste substances, and the discharge of liquid water from 

 plants as underlie the absorption of dissolved substances. 

 Thus we have glands receiving from the other parts of 

 the plant body by diffusion and osmosis certain sub- 

 stances which they may so transform that they can accu- 

 mulate and retain them. Thus it is conceivable that the 

 glands on the surface of a geranium leaf receive from the 

 leaf sugars of one or more kinds from which they make 

 and secrete the volatile oil which gives the characteristic 

 fragrance to the foliage of this plant. There are glandu- 

 lar hairs which excrete water, presumably because the 

 permeability of their outer cell-membranes to water ex- 

 ceeds the permeability of adjacent membranes, so that 

 the water will escape through the walls which are most 

 permeable to it. Such hairs occur on the leaves of 

 beans. 



GuTTATiON AND Bleeding. — When the pressure of 

 water in cells exceeds the ability of the cell membranes 

 to hold it, water will be squeezed out. This pressure of 

 water, or cell-sap, is known as turgor. At certain times 



