84 



really hot day must be very considerable. This, again, is obvialea 

 by a grassy covering which prevents excessive loss of water although 

 the grass itseU'is transpiring freely. 



GRASS TOXINS. 



Now there comes the question of root toxins, about which we 

 may say that as yet really very little is known. We know that in 

 the case of herbaceous plants rotation of crops is essential, that 

 plants of the same kind cannot be grown succcbsfuliy on the same 

 soil continuously, and we also know that seedlings of a big tree do 

 not thrive beneath the shade, or perhaps within the root-area of the 

 mother tree, whence nearly all trees climbers and other such plants 

 have special modifications of fruit or seed to disperse the seed from 

 the parent. These facts are accounted for by the theory that plants 

 excrete from their roots a poison or toxin. 



The most important contribution to our knowledge on this sub- 

 ject is the paper by F. Fletcher in the memoirs of the department of 

 agriculture in India, Vol. II. No. 3, " note on a toxic substance 

 excreted by the roots of plants." He worked exclusively with her- 

 baceous plants, cotton, sorghum, wheat, etc. Hi-s experiments tend 

 to show that the toxins of different plants are identical, and that 

 solutions made of them differed in toxic effect only from their strength 

 and not from their kind. 



If then all plants produce through their roots a toxin poisonous 

 to other plants, this toxin being the same for all plants it is impos- 

 sible to see how a grass plot or forest can exist f'^r more than a few 

 years. As soon as a forest tree had produced a certain amount of 

 toxin everything in the neighbourhood of its roots should die. If 

 each plant had its own toxin which was poisonous to its own species 

 only, it is not clear how one tree forests, such as the pine forests of 

 Europe, or coconut tree estate could exist at all, nor does the sugges- 

 tion that in the case of sorghum and cotton grown on the same soil 

 the roots of cotton godown below the sorghum root area and the 

 toxin is retained by the roots of the sorghum when decaying and thus 

 the cotton is not affected after the death of the sorghum. 



In a grassplot and in a forest the roots of the different plants are 

 intimately mixed in the same layer of soil, and yet the plants grow 

 healthily together and the chief struggle between them seems to be 

 for light, and soil food. 



The whole subject is very puzzling and dithcult to understand, 

 ' and it is clear that much more research is required to clear up these 

 points. 



What is really wanted is some system by which we can, while 

 avoiding all injury caused by toxins of grass or other herbs, to pre- 

 vent the excessive loss of soil and plant food by denudation, and loss 

 of water and root injury by the great heat of the sun striking on the 

 bare soil. — Ed. 



