496 ORGANOLOGY. 



free water, and applied their results to plants growing in the earth. In 

 the next place, it is necessary to ascertain far more accurately than has 

 hitherto been done, in what circumstances the water is retained in the 

 various soils, and most especially in that form of it called humus. That 

 a difference exists is shown very evidently by the differences existing in 

 the root of a plant, when grown in dry earth and in water. In the last 

 case, the entire surface is smooth ; in the first, the cells of the epidermis 

 are more rapidly developed in proportion to the dryness of the earth, and 

 form long papillae which are insinuated around the smallest lumps of 

 earth. The cells of the roots of plants growing in water consist of pro- 

 portionably broad cells, the contents of which are exceedingly thin. On 

 the other hand, in plants growing on the land, those parts which take up 

 the nutrient matter are composed of a very delicate small-celled tissue, 

 the contents of which is mostly mucus, which consequently exerts a very 

 strong endosmotic action. This or a similar difference shows, that if the 

 nutrition of the plant is effected through endosmose, that in this case it 

 has to overcome, not only the power of attraction in the mixture, but also 

 the power with which the constituents of the soil retain the absorbed 

 water. In this case the experiment is required to determine what 

 difference takes place in the endosmometer, between the action of a 

 diluted fluid by itself, and the same mixed with a quantity of mould. 



In recent times we have received important additions to our know- 

 ledge of the physical peculiarities of the substances contained in the 

 soil, and have learned consequently to regard them in quite another 

 light. Generally, the soil consists of various kinds of rocks, decomposed 

 and disintegrated, and also of a quantity of soluble and insoluble, more 

 or less easily decomposable inorganic combinations, mixed with a smaller 

 or larger proportion of organic substances in a state of decomposition. 

 These various organic and inorganic combinations possess, in a very 

 varying degree, the properties of forming looser or more compact masses 

 amongst themselves, of retaining water or allowing it to pass freely 

 through them, of condensing the vapour of the atmosphere, absorbing 

 carbonic acid, oxygen, and ammonia, &c. On these various properties 

 depend, universally and essentially, the fruitfulness of a soil, in so far as 

 it depends on the greater or less facility with which the nourishment of 

 plants is taken up by the process of endosmose. In this relation, the 

 half decomposed organic substance, known by the collective name of 

 humus, is important, as it possesses the properties of absorbing gases and 

 vapour, and retaining moisture for a long time. Wood-charcoal also 

 appears to possess properties of this kind, worthy of investigation ; and 

 according to the experiments of Lucas, it would appear to be especially 

 beneficial to certain kinds of plants. It is on account of this substance, 

 apparently, that certain plants, such as Marchantia polymorpha and 

 Funaria hygrometrica, are almost sure to spring up on the spots where 

 wood fires have been kindled, and their ashes left on the ground. 

 Special researches on this point are demanded by agriculture and 

 horticulture. 



The third case mentioned in the paragraph is, I willingly admit, 

 somewhat hypothetical. When we examine the tropical Orchidacece, as 

 they grow luxuriously in our hothouses, and find that only one or two of 

 their roots adhere by their sides to the bits of cork on which they grow 

 suspended, and consider that the peculiar covering of their roots dis- 

 tinguishes them from all other roots, and that this is composed of a spongy 

 cellular tissue, resembling in its physical properties those bodies which, 



