38o SCIENCE PROGRESS 



be infected and thus indicates some structural feature as the 

 cause of their natural immunity. T. McLean {Bull. Torrey 

 Bot. Club, April) has examined the stomata in a highly resistant 

 and a very susceptible species and finds that in the former the 

 pore is narrower than in the susceptible Citrus. By preventing 

 access of water to the ante-chamber this may well account for the 

 difference in susceptibility to disease. 



Miss Rea {New Phyt., June) finds that the stomatal frequency 

 in Cmnpanula rotundifolia increases with increased height of 

 the leaf above the ground, thus agreeing with Yapp's observa- 

 tion on Ulmaria palustris. The number of hydathodes present 

 on the upper surface of the leaves was greatest in the shade 

 shoots, where as many as 99 were found on a single leaf. In 

 the shade plant the hydathodes were more numerous on the 

 upper than the lower leaves, but in normal illumination and 

 fully insolated plants the uppermost leaves bore an apical group 

 only. 



Ecology. — Dealing with the ecology of Urtica dioica, Olsen 

 {Journ. Ecology, September) finds that 10-20 per cent, of open 

 sunlight is the most favourable condition of illumination, but 

 that intensities as low as 5 per cent, are tolerated. Whilst 

 favoured by moisture this species cannot endure water-logging 

 of the soil in winter. In common with its most frequent 

 associates Mercurialis perennis, Stachys sylvatica and Chrysos- 

 plenium alternifolium, Urtica dioica is rich in nitrates, showing 

 that these are present as such in the soils on which it occurs. 

 The most vigorous plants were found where nitrification was 

 most active, and this occurred in the damper soils, but showed 

 no correspondence with the acidity. The amount of phosphoric 

 acid also appears to be an important factor. 



In the same journal Salisbury and Tansley give an account 

 of the Quercus sessiliflora woods of the Malvern area, some of 

 which occur on soils derived from highly calcareous strata. It 

 is shown that this is not inconsistent with the normal occurrence 

 of this tree in Britain, since the surface layers in which the 

 seedling is rooted at the critical period are almost completely 

 leached, and, even on the soils derived from the Wenlock Lime- 

 stone, may exhibit an appreciable acidity. The ground-flora 

 is intermediate in character between that of a typical Quercetum 

 sessiliflorce and that of the Oakwoods of calcareous soils. The 

 woods on the May Hill Sandstone, the Malvernian, the Aymestry, 

 and the Wenlock soils show a progressive increase of basicity 

 accompanied by an increase of species normally confined to non- 

 acid soils. In the more acid woods these are confined to the 

 flushes, but even on the Wenlock Limestone the marked leaching 

 of the flat wood surface results in the partial restriction of these 

 species to the regions of maximum erosion {e.g. path sides, etc.). 



