THE LIFE -HISTORY OF PLANTS. 



99 



of absorbing and, retaining water, are in most cases ol 

 much, greater importance than its chemical compo- 

 sition; and hence, while the latter point is not to be 

 overlooked, nor the agency of manures regarded 

 as by any means unimportant, yet these are in most 

 cases secondary to the physical condition of the soil. 

 The delicate nature of the root-fibrils, their marvel- 

 lous powers of sensation and movement, also ilkistrate 

 this, and show the necessity for a proper relation be- 

 tween the nature of the root and the porosity of the 

 soil, its temperature, and power of absorbing water 

 and various gases, such, as ammonia. The care exer- 

 cised by the gardener in the construction of well- 

 drained j)orous vine-borders, and in the selection of 

 a[jpropiiate potting-soil according to the nature of 

 the plant and the character of its roots, is thus 

 abundantly justified. 



So, too, the character of root-growth and the 

 amount of root-activity largely depend on the tem- 

 perature and moisture of the soil. A hot, relatively 

 dry soil tends to promote succulent growth ; the plant 

 under such, circumstances is under the necessity of 

 storing water; and those that by peculiarities of struc- 

 ture resulting from hereditary endowment can do this 

 will be placed at an advantage. A porous, moist, 

 warm soil is the most favourable for the development 

 of fibrous roots and root-hairs, the actual amount of 

 heat, of course, varjang according to different plants. 

 A hot or water-logged soil would cause the root-fibres 

 to .rot and decay. A cold one would either not allow 

 of their formation, or, if formed, it would offer such 

 obstacles to their penetration that they would be of 

 little value, and gradually dw^indle. The power, 

 however, that roots have, under such circumstances, 

 of extending themselves for long distances in search 

 of a more propitious state of things, is one of their 

 most noteworthy properties. 



Soils consist either wholly or in various degrees 

 of admixture of one or other of the following 

 ingredients : — clay, sand, lime, or decayed vegeta- 

 ble matter (humus). The clay consists of aluminium 

 in combination with silicic acid, the sand con- 

 sists of insoluble silica, the lime exists in the form 

 of carbonate or phosphate of calcium. But these 

 substances are rarely pure and unmixed ; the clays 

 usually contain much potash, the sand also may 

 contain potash, or salts of lime, or iron, accord- 

 ing to its origin ; the lime is usually mixed with clay 

 or sand, or both. Of the humus, consisting of decayed 

 vegetable material, w^e shall have to make special 

 mention further on. Many of the substances named 

 as existing in the soil are, under ordinary circum- 

 stances, insoluble, and hence they are useless in that 

 form to the plant, while those that are soluble are 

 apt to be washed out hy heavy rains, and to find their 

 way into the water-courses by natural or artificial 



drainage. If the feeding-roots of plants are at hand, 

 and in an active condition, they may absorb some of 

 these soluble matters forthwith by the process of 

 osmosis, as previously explained, and in this way the 

 nitrates more particularly are taken up. 



Solution of Inert Materials.— There still 

 remain the insoluble matters in the soil, matters 

 so little, if at all, soluble in water, that the roots 

 would be unable to absorb them were th.ere not 

 some means of rendering them soluble. So far as is 

 known at present, there are two principal methods of 

 converting the insoluble and inert into the soluble 

 ^and useful, one external to the plant itself, by the 

 agency of bacterial microbes," the other dependent 

 upon the acid secretions of the plant itself. A few 

 words of explanation may be given in illustration of 

 both these phenomena. 



Bacteria. — Under the general name of " bacteria," 

 or the still more comprehensive name of " microbes," 

 are included a number of different forms, supposed to 

 be plants in the simplest stage of development, of ex- 

 treme minuteness, extreme simplicity of organisation, 

 biit with a prodigious power of multiplication under 

 propitious circumstances. They not only vary in size 

 and form, as above stated, but especially in their life- 

 conditions, the circumstances propitious to some 

 being indifferent, or even fatal, to others. It is by 

 the agency of germs of this nature, that various kinds 

 of fermentation are induced ; such as the alcoholic, in 

 which, after starch has been converted into sugar, a 

 fm-ther change takes place into alcohol and vinegar ; 

 the lactic, the butyric, &c. So certain diseases, fevers, 

 of various kinds, hydrophobia, diphtheria, cholera, &c. , 

 have been proved to be connected with, and in many 

 instances to bo absolutely dependent on, these minute 

 organisms. The proof is obtained by experiment, as 

 by isolating and cultivating the germs, and thus their 

 mode of growth and the changes they bring about in 

 animal or vegetable fluids may be ascertained. On the 

 contrary, by rigorously preventing their access, or by 

 destroying them, the changes in question do not take 

 place, and cannot be made to do so till the microbes 

 are again allowed access. In all probability we are 

 but on the threshold of these inquiries, and future 

 investigations will reveal that many of the phenomena 

 which go on in the living plant arc essentially con- 

 nected with these agents. Already it has been shown 

 (and this is our reason for alluding to them) that 

 certain of these microbes effect the conversion of in- 

 soluble and inert nitrites in the soil into soluble and 

 active nitrates. To explain how they do this would 

 necessitate a knowledge of chemical details which is 

 not I'equisite for our present purpose, which is simply 

 to point out one Avay in which inert matters in the 



