SECTIONAL TRANSACTIONS.— K. 421 



It is confirmed that the different salts of iron have different abilities to 

 catalyse chlorophyll formation, but an iron salt which is most efficient for 

 one species may not be the most efficient catalyst in another species. The 

 variations in chlorophyll content in different species, brought about by 

 different iron salts, are correlated with variations in dry weight. 



Zinc in small quantities helps to increase the amount of chlorophyll 

 developed, but beyond certain concentrations of zinc salt the amount of 

 chlorophyll decreases. High concentrations of zinc lead to orange-spotted 

 leaves or chlorosis. 



In concentrations of zinc salt which tend to induce chlorosis, the addition 

 of minute traces of manganese restores normal green coloration. 



Plants grown in shade or darkness and subjected to the action of a zinc 

 salt recover from light-chlorosis more slowly than controls. 



' Afternoon. 

 Mr. H. G. Chippindale. — The vitality of grass seedlings (2.15). 



A study of the behaviour of grass seedlings subject to interspecific com- 

 petition has led to an inquiry into their toleration of conditions inhibiting 

 growth. Their vitality has been found to be remarkable. 



A large proportion of seedlings survive desiccation for several days at 

 laboratory temperature, new growth being initiated in the shoot primordia. 

 The property is possessed in varying degree by different species, and may 

 also be affected by the previous history of the seedlings. 



The longevity of seedlings in complete darkness is dependent upon 

 temperature, but exceeds what would be expected from the food reserves 

 in the caryopses. At an average temperature of about 7° C, seedlings are 

 alive after several months without light, and on its admission are capable 

 of normal growth. Seeds germinating below the soil in autumn may not 

 show visible seedlings at the surface until the following spring. 



Both the intensity and quantity of light requisite for the growth of grass 

 seedlings are extremely small ; when below the minimum, survival of the 

 seedlings is protracted, and it is possibly dependent upon mineral nutrition. 



Dr. Winifred E. Brenchley. — The varying response of weed species to 

 competition (2.40). 



The competition of weeds with crops plays an important part in eco- 

 nomics, but the factors influencing the competition are not fully understood. 

 Probably in some cases the apparent association of certain species with 

 particular types of soil is in reality determined by the relative competition. 



When arable land is fallowed, the succeeding wheat crop tends to be very 

 heavy on account of the accumulated fertility, but in spite of this heavy 

 crop such weeds as Alopecurus agrestis may show a big increase, although 

 under normal cropping conditions wheat and Alopecurus come into active 

 competition. Weed species may be divided into three groups according 

 to their behaviour after fallowing : 



(1) Species which fail to reassert themselves in the face of crop competi- 

 tion. Papaver rhceas came into this group at Rothamsted, in spite of the 

 large number of seeds which survived fallowing. 



(2) Species which withstand the competition of the crop and replenish 

 their stocks of seed in the soil. 



(3) Species which respond variably after different periods of fallowing. 

 Extreme cases of competition are exemplified by Broadbalk and Gees- 

 croft ' Wildernesses.' In the former an uncut wheat field has reverted to 



