13 



IV. PLANT NUTRITION PROBLEMS. 



A considerable portion of Dr. ii.cnchley's time has been taken up 

 in studying the effects on plant growth of various substances to test 

 possible fertilising values^ this information being wanted for the Food 

 Production Department. Her ordinary work has^ however^ been 

 maintained; the investigations on weeds have been extended; a com- 

 plete botanical analysis of the herbage of the grass plots has been made 

 and will be examined in detail; also an ecological study has been 

 made of the recolonisation of arable land allowed to revert to natural 

 conditions — the cases examined being Broadbalk and Geescroft Wilder- 

 nesses, which went out of cultivation in 1881 and 1882 respectively. 



A problem of importance in water culture work has been further 

 studied. It has been shown that the amount of plant growth is related 

 to the concentration of the nutrient solution and increases with it to a 

 maximum set by the other conditions. 



The effect of certain organic toxins, especially cyanides and phenols, 

 on plant growth has also been studied. This investigation was necessi- 

 tated by the circumstance that some of these substances are of consider- 

 able interest as possible insecticides and partial sterilisers, and it is 

 important to know how they are likely to act on the young plant. The 

 information given by water culures is not complete and needs checking 

 by direct studies in soil, but so far as it goes it has the great advantage 

 of simplicity and freedom from complication. 



The investigations on the sugars and starch in plants begun by 

 Mr. Davis in 191 1 in conjunction with Messrs. Daish and Sawyer were 

 continued by him until 1915 when he took an appointment under the 

 Indian Government ; they were then handed over to Mr. Horton. 

 Considerable attention was devoted by Mr. Davis to the leaf of the 

 mangold, and samples were taken at regular intervals for analysis. 

 Starch and maltose are entirely absent from the leaf after its earliest 

 growth. In the early stages saccharose occurs in excess of the hexoses 

 in the leaf, but later in the season, when sugar is being stored in the 

 root, the hexoses exceed the saccharose in amount. In passing from 

 leaf to root the proportion of hexoses greatly increases ; in the midribs 

 and stalks the hexoses always predominate and their predominance 

 becomes more and more pronounced as the season advances. 



The results indicate that saccharose is the first sugar formed (as 

 Brown and Morris have already shown), and that it is not carried to the 

 root as such, but changed into hexoses for the purposes of transit, and 

 then changed back again to saccharose in the root. The mechanism of 

 the change in the root was not discovered. 



Apparently, similar conclusions apply to other plants, the vine, 

 potato, dahlia, etc. 



The absence of maltose was very carefully confirmed ; over 500 

 analyses of various leaves and germinating seeds were made, but in no 

 case was any trace of maltose found, even where starch was being 

 broken down and where therefore maltose must have been formed. 

 This is attributed by Messrs. Davis and Daish to the widespread 

 distribution of the enzyme maltase which breaks down the maltose at 

 once to glucose. The degradation of starch, in their view, involves at 

 least three stages :" the transformation of starch to soluble starch and 

 dextrines, brought about by special Hquefying enzymes ; the conversion 

 of dextrines to maltose by the enzyme dextrinase ; and the conversion 

 of maltose to glucose by maltase. 



