CHAP. II. 
POLLEN. 
189 
them is not observable: but in Taxus, Juniperus, Cupressus, 
Thuja, the outer membrane has, as Mohl states, so little ex- 
tensibility, that it is torn irregularly, and the inner mem- 
brane protrudes beyond the crevices, and, swelling more and 
more, generally disengages itself from the extine. It some- 
times happens that the inner membrane protrudes beyond the 
outer shell, in the form of a short sac or tube : this phenomenon 
may be produced artificially]at will by placing the pollen-grain 
in weak nitric or sulphuric acid ; but it is quite a distinct emis- 
sion from that of the pollen-tubes hereafter to be noticed. 
A third membrane, intermediate between the extine and 
intine, was first noticed by Mohl in the pollen of Taxus, 
Juniperus, Cupressus, and Thuja. Fritzsche calls it the Ex- 
intine^ and finds it not only in these plants, but also in Pinus, 
Cucurbita Pepo, and Tigridia Pavonia, and considers it pro- 
bably a common structure. The same minute observer 
speaks of four coatings to the pollen of Clarkia elegans, call- 
ing the fourth, which is next the extine, the Intexine ; he also 
finds the same structure in other Onagraceag. 
Mohl names Asclepiadaceous plants as those only in which 
pollen has but one tunic ; but Fritzsche asserts that these 
plants have both an extine and intine, and he figures 
them in Asclepias syriaca; he adds, that in Caulinia fragilis, 
Zannichellia pedunculata, Zostera marina, and Naias minor, 
the pollen has really nothing but the intine present. 
There are few forms of pollen in which the extine presents 
the appearance of a vesicle completely closed. In many 
cases the grains are marked by a longitudinal furrow on one 
side, and look when dry like a grain of wheat. Mr. Griffith 
has shown, as is above stated, that this appearance is caused 
by a fissure in the extine, and if such pollen is put into 
water the fissure becomes less visible. The presence of one 
or more such clefts is of common occurrence ; and have been 
supposed to be openings through the extine down to the in- 
tine; Mohl, however, considers them not to be really open- 
ings, but only extremely thin spaces in the extine. Fritzsche 
calls them pores, and regards them as certainly openings. 
Instead of slits many kinds of pollen have circular holes, 
varying in number, in different species, from one to an in- 
