496 SCIENCE PROGRESS 



ground, so that it is evident that the grass-effect cannot be 

 explained by the presence of any excess of that gas in the soil. 

 Moreover, trees grown in soil into which a current of carbon 

 dioxide was led showed no alteration in behaviour. 



The temperature of the soil under grass is on the average 

 somewhat lower than that of tilled ground : though during the 

 night it is slightly higher, in the daytime, under favourable 

 circumstances, it may be as much as io° F. lower. But the average 

 day excess during the summer would be only about 3 and as 

 this is less than differences observed in comparing one season 

 with another, it is clear that it will not account for the action ot 

 the grass ; added to which the grass-effect is equally apparent in 

 the case of plants grown in pots in a greenhouse where the 

 temperature of the soil in the various pots would be practically 

 identical. 



The possibilities of the influence of bacteria on the results 

 have not yet been fully investigated but it is clear that the mere 

 number of these cannot be accepted as an explanation of the 

 grass-effect. The growth of grass is found generally to increase 

 the number of bacteria in the soil : in certain experiments, for 

 instance, the increase was from 2*3 to 9 million per gramme ; but 

 we may still have as great an effect of grass on the tree as 

 occurred in this instance, when the bacterial contents is as low 

 as 2 - 5 million, this being the case when the tree and grass are 

 grown in sand instead of in earth. 



The Question of Toxicity 



A review of the whole of the facts relating to the effect of 

 grass on trees can leave very little doubt that the action is due 

 to some toxic effect, at any rate when this term is used in a wide 

 sense. The tree is not deprived by the grass of the food or 

 water necessary for its welfare ; these may be present in abun- 

 dance but it is incapable of utilising them : this is characteristic 

 of a toxic action. Long before all the evidence here alluded to 

 was obtained, such a conclusion was the one arrived at and to 

 those who have had trees suffering from grass constantly before 

 them, during many years, it would be difficult to arrive at any 

 other. A toxic action, however, does not necessarily mean that 

 the grass-roots excrete some substance which is poisonous to the 

 tree : there is a considerable amount of debris from the roots oi 

 grass while it is growing, which on decomposition might form 



