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structural features even if they belonged to the most widely separated 
groups. This is only exceptionally true. Experience shows that one 
species reacts in one way, other species in a different way, under the action 
of the same stimulus, but that the reaction is often of the same kind in 
plants belonging to the same phylum. Thus one s,pecies protects itself 
against desiccation solely by means of mucilage receptacles, another by the 
development of hypodermal aqueous tissue, a third by enlargement of 
epidermal cells, and others by two or more of these features. Reiche, 
Volkens, and others have shown that climate and habitat do not impress 
any one definite type of anatomical structure upon all the species of a certain 
geographical area. Species possess a definite plasticity which, however, 
may vary in degree and direction in individuals; in such cases we may find 
discrepancies between structure and external conditions. According to 
Vesque and Areschoug the leaves of Nelumbium bear stomata on the upper 
side only, just like floating leaves. This fact is drawn from the theory that 
Nelumbium is derived from an ancestral form possessing floating leaves and 
is supported by the results of physiological researches in which it has again 
been emphatically shown that the anatomical structure is the product of 
two factors — adaptation and heredity. The second factor, which some- 
times becomes more noticeable than the first, allows us to employ biological 
structural features to a very considerable extent for systematic purposes. 
Biological characters serve principally for the diagnosis of species. 
Within the same group of affinity, these characters are often identical in all 
those forms in which they appear; or they may be constant for groups of 
allied species, for genera, or for small orders. Biological characters may be 
divided into those which differ qualitatively and those which differ quanti- 
tatively; of these the former have the greater systematic value. The 
presence of hypoderm in a leaf is a more important fact than the number of 
layers of hypoderm. Abundant material of the same species from different 
habitats and cultural conditions should be compared. 
Description of Leaves 
Gramineae 
Andropogon scoparius Michx. 
Habitat: Dry soil; hill crests; slopes. 
1 Orientation and arrangement: Blade ascending, appressed to stem when young; op- 
posite. 
Gross structure: Lanceolate; blade glabrant except near sheath. 
Histology (fig. I, plate IX): 
•Outer walls of epidermis twice as thick as inner; buUiform cells prominent. 
Parenchyma: Palisade cells concentric around the vascular bundles; spongy tissue 
between bundles; vascular tissue prominent. 
Stomata small. 
1 The term orientation as here used refers to the plane in which the leaf blade lies, 
whether horizontal, vertical, or ascending. 
