246 ASK CONSTITUENTS OF PLANTS. 



annual temperature of the atmosphere in temperate regions.* 

 Moisture exerts a very great effect in equalizing the capacities 

 of different soils for absorbing and retaining heat. 



663. That the saline matters in the soil must be in a form in 

 which the plant can make use of them, appears from what has 

 been said about osmosis. It should be specially noticed, how- 

 ever, that jounger roots may exert a solvent action upon soil- 

 particles. 



Root-hairs, as Sachs ^ has shown, evolve small amounts of 

 acid, which exert a distinctly corrosive effect upon certain min- 

 eral matters with which they come in contact. Hence tliere is a 

 continual unlocking of tlie nutritive mineral materials fastened in 

 the soil ; the release being at the very points where the root-hairs 

 are present to absorb them. 



ASH CONSTITUENTS OF PLANTS. 



664. These occur in all parts of plants. It has been shown 

 (p. 39) how frequently- cell-walls are impregnated or incrusted 

 b3' mineral matters, which after careful calcination ma^- be left 

 as a distinct skeleton of tlie tissues of which they formed a part. 

 But the matters within cells, both the protoplasmic substance and 

 the cell-sap, also contain a certain amount of incombustible ma- 

 terial. The total amount of ash constituents varies greatly in 

 different plants, in different parts of the same plant, and also 



1 Penhallow, Soil Temperatures (Hougliton Farm Experiment Department),: 

 1884. See also Knop, Agricultur-Cliemie, i., 1868, p. 469. 



2 Moldenhawer (Beytrage), in 1812, expressed the view that roots probably 

 set free certain matters which can unloose nutritive materials. De Candolle 

 (Physiologic, 1832) described the corrosive action of lichens on underlying 

 rocks ; and Liebig, in 1839, studied the action of roots on the color of litmus 

 solutions. 



Sachs's experiment (1860) is well adapted to class demonstration. A pol- 

 ished plate of marble is covered with moist saw-dust, and in this a few seeds are- 

 planted. After the seedlings have grown for a time the saw-dust is removed,- 

 when the marks left upon the stone by tlie corroding rootlets can be plainly 

 seen. If the corroded marble is rubbed slightly with a little vermilion, the 

 traces made by the root-hairs will be very distinct. In the early publication 

 of Sachs, the secretion by which the corrosion is effected was said to be car- 

 bonic acid ; but he does not,appear to hold this view now. Whether the action 

 is due to acetic acid, as Oudemann and Kanwenhoff suggest, or to different- 

 acids varying with plants or times, as intimated by Pfeffer, it is certainly 

 highly corrosive in some cases. In an experiment by Schulz, the rootlets of 

 germinating Leguiiiinosse and Gramineie exhibited a faint alkaline reaction; 

 (Journal fiir Praktische Chemie, Ixxxvii., 1862, p. 135). ; 



