1917] McCNAIR—FATS FROM RHUS 333 
the ripe fruit the fat appears in the principal tissue of the mesocarp. 
Fat is not found in the exocarp, the thin walls and the inner bound- 
ary of the mesocarp, the sclerenchymatous cells, the cells of the 
vascular bundles and their sheaths, and the parenchymatous 
sheaths of the resin passages: 
The presence of solid fat in the fruit cannot be detected before 
July. At the beginning of August fat formation is nearly com- 
pleted. The granulated layer of fat can be seen in the cell between 
the membrane and the protoplasm. This layer makes the lumen 
smaller, increases on the outside, and goes in between the already 
formed fat. Its granular form changes to striated masses. Before 
the formation of this fat in the fruit a progressive increase in the 
starch content is noticeable. Starch forms partly in the chromato- 
phores in the cell and partly in the cells. When the fruit cells are 
rich in starch the cells contain besides only granular protoplasm and 
nuclei. This starch gives a positive reaction with iodine. When 
fat formation is near completion no starch can be detected in the 
fruit. In fruits which have nearly completed their growth the 
resin passages are everywhere constricted by the growth of paren- 
chyma sheaths. From a consideration of these phenomena fat 
is apparently formed from starch and not from the resin-like 
poisonous sap. 
This view does not seem untenable, for it has been proved that 
in the storage foods of plants carbohydrates and fats are inter- 
changeable, and in certain cases carbohydrates are entirely replaced 
by fats. Starch is stored in potatoes and in the tubers of dahlia, 
and cane-sugar is stored in beet root; the seeds of the two former 
plants contain oil, while those of the beet are starchy. Although 
the grains of most grasses contain starch, some instances are known 
in which fatty oil is present instead (Phragmites communis, Koeleria 
cristata, etc.). In the cotyledons of Impatiens Balsamina amyloid 
is stored in the form of enormously thickened walls, while in other 
species of Impatiens the tissue of the cotyledon is thin-walled and 
oil is present instead of reserve cellulose. 
The change of carbohydrates to fats in the seeds of plants 
has been studied by Scumupt (11), LeCLerc pu SABLON (4), 
and others. These investigators have shown clearly that as the 
