398 MR. T. W. WOODHEAD ON THE 
Changes in structure are produced by varying degrees of 
exposure to which the species are subjected, the modifications 
being concerned largely with conserving water by checking 
evaporation, e. g. thick cuticle, reduced number of stomata, more 
compact mesophyll and hairiness ; or the development of water- 
storage tissues. In some cases, hairs may function as water- 
organs, e. g. Vaccinium Myrtillus; or the modifications may be 
such as to provide a means of eliminating excess of water, e. g. 
thin cuticle, increased number of stomata, increased leaf-surface, 
larger intercellular spaces in the mesophyll, &e. 
The tissues most susceptible are the epidermal and ground- 
tissues, and not only are those of the leaf-blade affected, but 
also those of the petiole and rhizome, e. g. Pteris. 
Modifications occur which are of mechanical advantage to the 
plant in aiding it to resist stresses due to wind in exposed 
situations. Others are of advantage in supporting relatively 
large leaves on slender leaf-stalks developed in sheltered situa- 
tions in deep shade, e. g. Pteris. 
I wish to express my thanks to Miss H. M. Sikes, who has 
rendered much assistance throughout the work. I am also 
indebted to Mr. W. E. L. Wattam for help in mapping the 
distribution of species; to Messrs. J. W. H. Johnson and 
Edward Lodge for the trouble they have taken in making 
analyses of soils; to Miss B. Lomax for her great assistance in 
the preparation of sections and drawings; also to Mr. F. O. 
Mosley for help in the preparation of the maps. 
T also wish to acknowledge my indebtedness to the Earl of 
Dartmouth and to Sir Thomas Brooke, Bart., for their kindness 
in permitting free access to the woods on their estates. 
