28 



PUBLICATIONS DURING THE YEARS 1918-20. 

 SCIENTIFIC PAPERS. 



CROPS AND PLANT GROWTH. 



I. Winifred E. Brknxhley. '" Some Factors in Plant 



Conipctitioji.'' Annals of Applied Biolog-y, 1920. Vol. 

 \T. pp. 142-170. 



The competition exhibited when plants of the same or different 

 species grow in juxtaposition is complex and includes : — 



1. — Competition for food from the soil. 2. — Competition for 

 water. 3. — Competition for light. 4. — The possible harmful 

 effect due to toxic excretions from the roots, if such occur. 



The general effect of competition (including 1, 2, 3 above) has 

 been studied in pot cultures, when a varying number of plants are 

 grown in the same bulk of soil. The effect of competition for light 

 was investigated by means of water cultures, in which a number of 

 plants each equally furnished with food and water, were crowded 

 together as closely as possible, while a similar set of plants was 

 given sufficient space to avoid the shading- of one plant by another. 



With limited food supply the dominant factor in competition 

 is the amount of food and particularly of available nitrogen. Other 

 things being equal, the dry matter produced is determined by the 

 nitrogen supply, irrespective of the number of plants drawing 

 thereon. 



W^ith limited food supply the 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 available 

 for building- up the tissues. 



(N.B. — " Efficiency Index " is the term employed by V. H. 

 Blackman to express the rate per cent, at which the dry matter of ji 

 plant increases during- g^rowth.) 



3. — 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. With barley the effect 

 of light competition is to reduce the number of ears ; to cause great 

 irregularity in the number of tillers produced ; to reduce the amount 

 of dry matter formed ; to encourage shoot growth at the expense of 

 root growth, thus raising- the ratio of shoot to root ; to increase the 

 variation in the efficiency indices of dry weight production of a 

 number of crowded plants, lowering- them on the average ; to de- 

 crease the power of the plants to make use of the food supplied to 

 the roots, as evidenced by the total quantity of nitrogen taken up 

 by similar numbers of plants when spaced out and crowded. 



4. — With adequate illumination (in barley) there is a tendency 

 towards the production of a standard type of plant in which the 

 relation between the number of tillers and ears, dry weights, effi- 

 ciency indices, and ratios of root to shoot approximates within 

 variable degrees to a constant standard. With overcrowding, this 

 ai)i)roximation entirely disappears. 



II. WiNiFRKi) E. Brkxchlkv. '' 0)i tlic Relations hetiveoi 



(Ifowtli and the E}ivironmental Conditions of Tempera- 

 ture and Brif^ht Sufisliine." Annals of Applied Bio- 

 logy, 1920. Vol. VI. pp. 211-244. 

 The amount of growth made by any crop in the field and tlie 

 rate at which maturity is reached are influenced by many factors. 



