534 
HERBERT C. HANSON 
changes as compared with plants growing in the normal environment : 
storage tissue, the fibro-vascular bundles, air-spaces, dry weight, ash 
and acid content of the leaves all decrease ; chlorophyll tissue increases ; 
chlorophyll cells become more isodiametric ; walls of epidermal cells 
become wavy; number of stomata per leaf increases, although the 
number for a unit area may be the same; and epinasty replaces hy- 
ponasty, so that the leaves grow at right angles to the stem. He be- 
lieved the air factors, not the soil factors, determined these changes, 
but that physical explanations do not always seem adequate to account 
for the changes. 
Brenner (lo) concludes from his anatomical and experimental 
study on Quercus leaves that modifications caused by the environment 
are hereditary and may develop into new species. 
Bonnier (7) selected about fifty species of perennial plants at 
Fontainebleau. Each plant experimented upon was split, one part 
planted at Fontainebleau, the other at Toulon. The plants set out 
at Toulon became like the wild plants surrounding Toulon in leaf and 
and wood structure and in external characters. 
Hansgirg (18) gives descriptions in detail of more than fifty types 
of leaves, with a discussion of their ecological advantages. 
Chrysler (11) compared the leaf structure of nine strand plants 
from the Atlantic coast near Wood's Hole, Massachusetts, and from 
the vicinity of Chicago. The leaves of the maritime plants were from 
less than once to a trifle over twice as thick as the inland plants. The 
increase in thickness was mostly due to increased palisade develop- 
ment. Greater compactness in tissue and increased thickness of the 
outer epidermal wall were found in certain plants. The amount of 
salt in the soil is given as the probable cause for the variations. 
Boodle (8) in his experimental study on the leaves of Pteris aquilina 
Linn, found that leaves grown in dry and exposed situations had a 
xerophytic structure while those grown in sheltered positions were 
more mesophytic. The former leaves possessed hypoderm, the latter 
had none and the palisade was poorly developed, or entirely missing. 
The same differences were found on leaves of the same plant, or on 
different parts of the same leaf. A plant that had been producing 
shade leaves in a moist greenhouse, produced sun leaves when placed 
in a garden. The mature structure of the leaf is not determined at 
an early stage in the leaf's growth. 
Copeland (13) found great variations in the shape and thickness of 
leaves on the same branches of various plants. 
