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Notes on the Influence of Light on certain dorsiventral Organs, 
By KATHARINE CLEVELAND BURNETT. 
(PLATE 297.) 
The influence of light on morphology and anatomy of dorsi- 
ventral organs comprises a most important part of plant physi- 
ology. It is necessary therefore to explain that the work set forth 
in this paper is of a very limited character, owing to the short time 
available in a laboratory course, so that the subject should be 
limited to the study of the influence of light on the morphology 
and anatomy of dorsi-ventral organs of two plants studied during 
the months of March, April and May. 
The influence of light on a plant may be negatively studied by 
watching its behavior when deprived of light. The results of this 
deprivation are morphological, anatomical and physiological, and 
are known as the phenomena of etiolation. 
Some of these results are well known and prominent, many are 
exceptional and disputed. 
As a general thing, we notice morphologically the lengthening 
of the internodes and the smallness of dorsi-ventral leaves; also, © 
that the leaves make a very small angle with the stem. 
Studied from an anatomical point of view, the vascular and 
thick-walled tissues are found to be much less developed in the 
internodes, and no differentiation of palisade and spongy tissue is 
found in the leaves ; this is almost a physiological point, being con- 
nected with the non-formation of green chlorophyll pigment. A 
green plant deprived of light forms no green chlorophyll pigment 
and even loses that which it already has formed. 
It has been stated by various authorities that a dorsi-ventral 
leaf owes its differentiation of palisade and spongy tissue entirely 
to the influence of light, and ¢at for a distinct purpose. Accord- 
ing to Vines,* «‘ palisade layers occur always beneath the epidermis 
of those surfaces which are directly exposed to the sun’s rays. If 
a plant be grown in the shade the palisade layers are imperfectly 
differentiated even if they can be detected at all. Development 
of palisade layers is clearly a peculiarity of leaves which are ex- 
posed to sunlight. Bright light promotes assimilation and oxida- 
* Vines. A Students’ Text-Book of Botany, 686-687. 
