303 



A summary ot' the work wliich was carried out with mustard 

 and l)arley provides the following-: 



'• Tlie mutual action of one plant on another when growing in 

 juxtaposition, usualily knowii as competition, is a very complex 

 phenomenon. 



"*' VThen tlie food supply is limited the dominant factor of 

 competition is that of food and in particular the amount of avail- 

 able nitrogen. Other things being equal the total growth as 

 measured by the dry matter jjroduced is determined hy the nitrogen 

 supply, irrespective of the number of plants drawing on the re- 

 ■sources. 



" With limited food supply tlie efficiency index of dry weight 

 production decreases with the number of plants as the working 

 capacity of the plant is limited by the quantity of material avail- 

 able for luiilding up the tissues. 



"The decrease in light caused by overcrowding is a most 

 potent factor in competition even when an abundance of food and 

 water is presented to each individual plant." 



T. F. C. 



Effect of Lightning on Trees. 



Tlie question as to what extent groups of trees are effected by 

 lightning and how far the damage extends after they have been 

 struck is frequently discussed on estates where apprehension is 

 often felt as to how wide a circle from the tree actually struck will 

 be affected. The following extract taken from the Indian Forester 

 A"ol. XLVI, 1*^0. 3. contains interesting observations on this subject. 



" Lightning-strnck trees may be found surrounded by others 

 which show no signs of having been struck at all, and trees standing 

 only 4 ft. away from a tree may thus escape. On the other hand 

 several trees standing close together are usually all more or less 

 similarly affected. Of a number of records which I have of the 

 maximum distance apart of anv two trees struck in the same 

 locality the four greatest distances are 50", 36", 35" and 33". 



"■ Young chir advance growth and. small woody shrubs have 

 been found killed within a circle up' to 18 feet radius round the 

 base of a lightning-struck tree, but it is more frequent to find such 

 shrubby growth apparently unaffected and I have no Tecord of her- 

 baceous growth showing any signs of damage at all. It is of course 

 well known that the taller an object is the more likely it is to be 

 struck, and it would therefore be imnatural to expect to find shrub- 

 by growth affected to the same extent as trees standing overhead. 



" As a matter of fact I believe that only a very small per- 

 centage indeed of trees struck w^ould die if other agencies did not 

 combine to complete their destruction. Overmature tree with de- 

 creasing vitality might succumb but not healthy sound trees in full 

 vigour. From general observations which are not, however, based 

 on definite countings, I believe that under existing conditions about 

 50 per cent of trees struck (namely, so severely as to give clear 



