IOWA ACADEMY OF SCIENCES. 
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such a manner that the cells which occupy the place of the sub- 
sequent receptacle becomes filled at an early period with the 
respective secretion. Afterwards the membrane of these cells 
containing the secretion disappears. 
DeBary states that in Dictamnus, beneath the epidermis, 
but derived in part from it, there arise cavities containing 
ethereal oil. 
The presence of oil cavities is a general and characteristic 
phenomenon in members of the order Eutacece. In the stem of 
Dictamnus and Correa these cavities lie directly under the epi- 
dermis. 
Their origin is in all cases lysigenetic. He refers to a des- 
cription and drawing by Frank of Ptelea trifoliata. The oil 
glands of Citrus are located in the yellow part of the rind, 
sometimes extending into the white layer. They are large 
structures visible to the naked eye, the walls are composed of 
thin walled rectangular cells so arranged as to give the glands 
an oval appearance. In the very large oil spaces in the rind 
of the fruit of Citrus a solution of the cell walls is distinctly 
perceptible. 
This is probably still more the case in trunks of Copaifera, 
in which the balsam passages attain an enormous development. 
These trees contain the Copaiva balsam in canals which are as 
much as an inch in diameter and which often traverses the 
entire trunks. 
Hop-tree differs from most of these in that the cell contains 
one large oil globule and several smaller ones. 
The resin of Ptelea is not soluble in cold water, but slightly 
so in warm water. It seems to dissolve in ether, but there 
remain small granules, 
In alcohol the resin dissolves and the globules break up. 
It is not soluble in glycerine. The common hop ( Humulus 
lupulus) and hemp (Cannabis sativa), although very different 
in structure, have similar resinous secretions. 
The fruit of the hop as well as the axis and the base of all 
the leaf-like organs are beset with numerous shining translu- 
cent glands to which the aromatic smell and taste are due. Hops 
found in commerce consist entirely of fully developed cones, 
more or less compressed. 
Each grain is originally attached by a very short stalk. 
The glands are short stalked, cupped plates of tissue, the cuti- 
cle of which is raised up by the bulky secretions, while the cells 
