The Royal Society, London. 373 



During the summer of 1899 some preliminary experiments were 

 made in Order to investigate the condition3 determining the formation 

 of certain outgrowths of which the structure had previouBly been 

 examined. *) These outgrowths conaist chiefly of greatly enlarged and 

 multiplied epidermal cells, with very thin walls ; but the underlying 

 parenchyma is often also affected. The cells concerned always lie imme- 

 diately around a stoma, so that the guard-cells are lifted up as the 

 outgrowth developes. The distribution of the outgrowths is there-fore 

 dependent upon that of the stomata, and they are pathological in origin 

 and nature. 



This year (1900) further experiments have been uudertaken, which 

 confirm and extend the conclusions suggested by the earlier work, and 

 which show that we have here a clear case of a pathological pheno- 

 menon brought under control. 



The plants used were chiefly Hibiscus vitifolius, but some obser- 

 vations were also made on Ipomea Woodii. 



The experiments wore designed to test the effects of moisture and 

 light in inducing the formation of the intumescences, but they also ser- 

 ved to show the influence of temperature. Most of them were made in 

 the open air, as the outgrowths always arise on plants growing in a 

 greenhouse. 



I. In order to test the effects of moisture in the air and in the 

 soil, plants were kept with their shoots in dry or moist air, and their 

 roots in dry or damp soil. Various combinations of dry or damp air 

 or soil were used, with the result that outgrowths were always formed 

 in damp air (provided there was sufficient light and heat), wbereas damp 

 soil had no effect. 



II. The effects of light were tested by growing plants in white light 

 of varied intensity, and under glass of different colours. Outgrowths 

 were developed under clear and whitewashed glass, and under red and 

 yellow glass, but not under blue or green glass, nor in poor light, and 

 never in darkness. 



III. Observations as to the influence of temperature showed that, 

 given the other necessary conditions, the formation of outgrowths is, 

 promoted by heat. 



Large outgrowths may be artifically induced with certainty in about 

 two days on a single healthy branch (still attached to the plant), by 

 isolating it in a damp atmosphere, and exposing it to a strong light at 

 a high temperature. 



The following is a brief summary of the präncipal experiments and 

 conclusions: — (Vergl. p. 374, 375.) 



The conclusions drawn from the above experiments are, that the 

 •outgrowths are formed in a moist atmosphere, provided that there is 

 also adequate light and heat. 



*) Dale, „On certain outgrowths (intumescences) on the ureen parts 

 of Hibiscus vitifolius Linn." (Proc. Catnb. Phil. Soc. Vol X. Part. 4) 



