THE DESCENDING SAP 129 



Thus we see how important a part the leaves play in connection with the 

 iipward flow of sap. Transpiration, which is carried on chiefly through the 

 stomata, not only gets rid of the superfluous water, but sets up a rapid 

 movement of the crude sap from the root to the leaves, drawing it upwards, 

 somewhat as the oil is drawn upwards in the wick of a burning lamp. This 

 giving-off of water by plants is often of considerable benefit to the regions 

 in which the plants are found. " It is a well-known fact," says Dr. Nathaniel 

 Ward, the inventor of the Wardian case, " that many hilly countries have 

 been rendered quite sterile, in consequence of the indiscriminate destruction 

 of their trees, the roots of which, taking up more water from the deep-seated 

 springs than the plants require for their own use, distil the surplus through 

 the leaves upon the ground, forming so many centres of fertility. ' Spare 

 the forests, especially those which contain the sources of your streams, for 

 your own sakes, but more especially for that of your children and grand- 

 children.' " * Needless to add, the quantity of water given off in the manner 

 described renders the solutions denser in the leaves 

 than in the stems a point that will come before us 

 again presently. 



Before leaving this subject of stomata we 

 should call attention to the analogous structures in 

 the bark, known as lenticels. As the stem or branch 

 of a woody plant grows, the epidermis with its 

 stomata gets too small for the increasing diameter of 

 the part. It cracks longitudinally and dies, becom- 

 ing dead bark, but connection between the air out- 

 side and the intercellular spaces of the cambium 

 within is maintained by means of the lenticels, FlG - 164. STOMATA. 

 through which carbonic acid gas passes outwards ! ^bto^ori^-uL 

 and oxygen inwards. These lenticels may be noticed three stomata - 



on the twigs of trees as little prominences, differing in tint from the sur- 

 rounding bark. In the Birch and Cherry they are especially noticeable as 

 transverse lines. 



Much more devolves upon the leaves than the giving off of superfluous 

 moisture. We have seen that the crude sap, which contains in solution the 

 nutritious principles, undergoes but little change during its passage from 

 the root to the leaves ; and also that the substances thus introduced into 

 the plant are, without exception, inorganic compounds. Yet these compounds, 

 if they are to be of any service to the plant, must be converted into organic 

 matter, in order that this, in turn, may form the plastic material or protoplasm 

 out of which new vegetable structures, such as cells, vessels, etc., may be 

 built up. In other words, the food must be assimilated by the plant ; and 

 this necessity pertains not merely to the nutrient substances absorbed from 

 the soil, but also to the carbon dioxide derived from the atmosphere. 

 * On the Growth, of Plants in Closed Cases, etc., pp. 10, 11. 



12 



