70 
ADA HAYDEN 
Methods 
Free-hand sections were made from typical mature leaves, usually se- 
lected from different plants, and from these the more representative sections 
were selected for mounts. After cutting, the sections were killed and fixed 
by immersion in hot alcohol, stained with a water-soluble safranin followed 
by haematoxylin, cleared in cedar oil and clove oil, and mounted in Canada 
balsam. Drawings of the leaves were made by camera lucida to a single 
scale of magnification. Qualitative data have been sought rather than quanti- 
tative. The thickness of the epidermis of leaves of one species as compared 
with the thickness of epidermis of another is not necessarily a positive or 
negative indicator of xerophytism, for one leaf may have trichomes plus a 
degree of thickness of epidermis found in the trichomeless leaf. Data 
concerning features regarded as "indicators" have been collected and rela- 
tive proportions of tissues have been noted in 'individual plants. 
Historical 
While many physiologists are yet skeptical concerning the use of such 
terms as "reaction" of a plant to its habitat, "modification," or "adapta- 
tion," careful investigation has shown that species do change their structure 
under different environmental conditions, although these words have been 
somewhat inexactly used and perhaps more exact terms might be evolved. 
Whether certain histological characters are due to environment or to ances- 
tral influence can be determined only by some histological knowledge of 
closely allied'forms and some experimental evidence regarding one species 
placed under different conditions. Schimper (15), who has done extensive 
research of recognized excellence in histology, ecology, and plant geography, 
as well as some work in physiology, states: "All experiments have led to 
essentially similar results. External conditions which, either by diminish- 
ing the absorption of water or by accelerating its exit from the plant, disturb 
the equilibrium in a sense hostile to the plant, occasion, as a rule, the fol- 
lowing deviations from normal structure: (i) Reduction of surface, the 
volume being assumed constant. (2) Diminution of intercellular spaces 
containing air. (3) Augmentation of vessels and sclerenchyma. (4) 
Lengthening of the palisade cells, frequent but not universal. (5) Increase 
in the thickness and amount of cutin of the outer wall of the epidermis. 
(6) Sinking of stomata. (7) Increased number of air-containing cells. 
(8) Supply of water-storing cells, such as double epidermis, aqueous tissue, 
mucilage cells. 
Haberlandt (6) calls attention to two main principles of the anatomical 
structure of the photosynthetic system: {a) the principle of maximum ex- 
posure of surface, and (h) the principle of expeditious translocation. 
The morphological structure of photosynthetic cells whose modifications 
are regarded as facilitating translocation are summarized by Haberlandt 
