sa7 
tion and decomposition of chlorophyll. Palisade tissue affords 
protection from the latter effect. When there is diffuse daylight 
the chlorophyll corpuscles are arranged horizontally along the 
upper surface of the cells. In direct sunlight they are vertical. 
The elongated form of the palisade tissue facilitates this with- 
drawal. The spongy tissue is especially adapted to transpiration, 
so in submerged water plants we find no palisade nor spongy tis- 
Sue. First, the light is not intense. Second, they do not tran- 
spire.” 
Furthermore, MacDougal, in his Experimental Plant Physiology,* 
says that if young leaves of beech be turned so that the morpho- 
logical under surface becomes the upper, the palisade will be found 
on what was originally the under surface. 
The following investigations were made for the purpose of 
verifying these statements, and, furthermore, to see if the tissues — 
of the leaf could be changed after the anatomical differentiation 
had taken place, that is, after the palisade tissue had formed could 
the leaf then be turned and palisade tissue be formed on the side 
now exposed to light. 
The plant experimented with was the Sale alba. Young 
twigs were obtained April 17th with buds % cm. long, slightly 
swollen but still covered with scales, the leaves, therefore, were not 
exposed directly to the influence of sunlight (a). 
On cross-sectioning a bud the tissue of the leaves showed 
regularly arranged parenchymatic tissue with no differentiation of 
palisade or spongy (b). 
The twigs were put in water, some in the dark and some ex- 
posed to light, to see whether the willow showed any exceptions 
to the ordinary facts of the etiolation. Ten days later, April 27th, 
the willow grown in darkness showed well marked etiolation. 
The terminal bud had grown 3% cm., being now 4 cm. long, the 
the lateral buds only from 1-2 cm. The internodes varied in 
length from 4% cm. tot cm. The leaves were small, from % to 34 
cm.in length; they made a small angle with the stem and had 
no green chlorophyll pigment. 
On cross-sectioning, the anatomy had changed but little from 
that of the bud studied ten days earlier. There was a very slight 
* Pp. 73-74. 
