340 
PHYSIOLOGY 
The problem, therefore, is: How can solutes pass the ectoplast usually 
impermeable to them ? The answer is merely in the form cf a hypothe- 
sis, like the one already proposed to account for 
guttation and bleeding. If the accumulation of 
the solute causes a rise of turgor, it is conceivable 
that the very pressure itself might work such a 
change in the cytoplasmic membranes that they 
alter their permeability and permit the outrush 
of water and its solutes in the direction of least 
resistance, which will be toward the free surface. 
Whether a renewed secretion will take place de- 
pends on the further activity of the cell. Given 
a repeated formation of the secretion, it might 
escape again. The hypothesis then suggests a 
rhythmic variation in the permeability of the cell 
membranes, the secretion being formed inside the cell. 
FIG. 636. Young 
resin gland of fir (Abies): 
a, duct, an intercellular 
space formed by the sepa- 
ration of the four nucleate 
cells. After TSCHIRCH. 
This hypothesis is clearly inapplicable to secretions which are not miscible with 
water, like essential oils and resins. They are probably formed, however, in the very 
wall itself, and thus the material 
may not have to traverse the ecto- 
plast as resin or oil. Unfortunately, 
even the place of their origin is still 
obscure. 
R61e of certain secretions. Nec- 
tar is gathered by many insects, 
some of which store it, after partial 
digestion, as honey. While the 
floral glands are being explored for 
nectar, the visitors become dusted 
with pollen and transfer this to ripe 
stigmas of the same or other flowers, 
thus insuring pollination in many 
cases where otherwise it might not 
occur (see Part III on pollination). 
The role of extrafloral nectar is not 
clear. Digestive glands, most defi- 
nite in insectivorous plants (p. 386), secrete enzymes (p. 399) by which the soft parti 
of captured insects are dissolved. Essential oils (p. 413) sometimes prevent plants 
from being eaten by animals. 
FlG. 637. Oil receptacle (a) in orange (Cit- 
rus Aurantium), formed partly by splitting, but 
chiefly by destruction of secreting cells and their 
neighbors (t) ; o, o, drops of essential oil. 
After TSCHIRCH. 
