310 Beer, Development of the pollen grain and antlier of sorne Onagraceae. 
Das only been found to occur in tlie few isolated cases mentio- 
ned above, otlier less extreme instances belonging to the same 
category of phenomena are not unknown. Wherever ve Und 
that a new lamella is interpolated between the protoplast and 
an older lamella and the latter still continues to grow in thick- 
ness or in snrface it does so whilst it is neither in union or in 
contact with the living element of the cell. 
Meanwliile changes are taking place in the protoplast and 
we find that a fluid is forming in the cytoplasm, partly at the 
expense of carbohydrates which have reached it fromwithout 
and partly at the expense of the cytoplasm itself. This flnid is 
forraed in vacuoles which gradually run together until the proto¬ 
plast is reduced to a liollow sphere enclosing a single, large, 
central vacnole. We have reason to believe that this fluid dif¬ 
fuses out from the protoplast and furnishes material for the 
growth of the pollen-membrane. 
The three most important features in the formation and 
development of the layers of the pollen wall may, therefore, be 
summarised as follows: 
I. Both the primary pollen wall and the secondary thickening 
layer originate in intimate connection with the plasmoderma 
(Hautschicht). 
TT . The greater portion of the subsequent growth of both 
these membranes takes place by intussusception whilst they 
are completely separated from the protoplast, 
III. The material required for the growth of the membranes 
is derived from the tecretory activity of the pollen-proto- 
plast. 
We can at present only vaguely guess at the most pro¬ 
bable way in which the growth of these membranes takes place. 
There are some facts, such as Ambronn's work 1 ) upon 
the optical properties of the cuticulariced walls, whicli indicate 
that the cell-wall may be underlaid by a crystalline structure 
and it is possible that when the membrane is first formed the 
protoplast* (to which it is then firmly fixed) determines the 
character and the arrangement of these crystals. 
The later growth of the membrane, even after it has become 
separated from the living element of the cell, may be considered 
to take place in a manner which depends upon the nature and 
relative positions of its crystalline components. 
8. After the pollen protoplast has become almost com¬ 
pletely exhausted by its secretory activity its substance is once 
rnore repenished by the material derived from the disintegration 
of the tapetum. 
The very young tapetum contains a ratlier scanty cytoplasm 
and only a single nucleus. Later the tapetal cells are furnished 
with a denser cytoplasm and nuclei which may vary in nuniber 
from one to eight. Until the end of the special-mother-cell 
i) Ambronn, H. Ber. d. Deutsch., bot. Gesell. 1888. p. 226. 
