242 



poor in vegetables ; it only brings forth the most formless and use- 

 less plants. Lastly, the garden soil not only nourishes in luxuriance 

 every plant that is committed to it, but even continually multiplies 

 the abundance of vegetable forms to infinity, to which, however, op- 

 posing climate sets a limit so soon as the favouring influence of cul- 

 ture is withdrawn. Then two other conditions present themselves, in 

 contrast, to our consideration. We have on the one side the common 

 soil, possessing little or no organic remains, and abundance of plants; 

 on the other, the bog and garden soils, both rich to superabundance 

 in the black constituent called humus, which has been formed by the 

 decomposition of animal and vegetable organisms. And nevertheless, 

 we find such a difference of influence on vegetation between the bog 

 and garden land. But this is readily explained by the manner in 

 which they have been formed. The peaty soil originates from the 

 decomposition of organic substances in the presence of much water. 

 The consequence of this is, that the water takes up and carries away 

 all the soluble salts which were contained in those organisms, so soon 

 as ever they are set free. In the garden soil, on the contrary, all 

 those soluble salts remain behind, come immediately into the posses- 

 sion of the plants, and, under a rich culture of the soil, accumulated 

 in them to an extraordinary degree, while the organic constituents, 

 through uninterrupted decomposition, are continually diminished in 

 quantity, and so can never accumulate in the way they do in the peat 

 or bog soils, where the presence of water, after a certain time, re- 

 strains or very much retards the further progress of decomposition. 

 A more striking proof of the correctness of the new views of the nutri- 

 tion of plants could not easily be given, than these statements ; views 

 which were almost simultaneously established and made known by 

 one of the most distinguished chemists, Liebig, and one of the most 

 eminent and practical agriculturists, Boussingault." — p. 170. 



In the eighth lecture — " On the Milk-sap of Plants," — the author 

 enters into some interesting details relating to three great families of 

 plants abounding in milk-sap. These families are the Euphorbiaceae, 

 the Apocynaceae, and the Urticaceae ; the latter order, however, has 

 been properly broken up, the milky plants formerly included in it 

 being now grouped together in a new order, the Moraceae, and to 

 these most of the author's observations more strictly apply. The 

 milk-sap of all the plants included in these orders contains more or 

 less of caoutchouc, which occurs in the form of little globules. These 

 are prevented from coalescing by an albuminous substance, in the 

 same way as are the butter-globules in milk. Exactly like the cream 



