204 Beer. — Studies in Spore Development . 
gained from the study of my preparations is that the clear space which 
is seen in the sections to lie between the thickening bands and the outer 
lamella represents a third and distinct layer of substance (with little 
affinity for stains) deposited by the protoplast previous to the development 
of the thickening bands. This layer subsequently becomes differentiated 
into the spines and rodlets. This interpretation of the layers of the exine 
of Ipomoea would be more or less in accordance with the views expressed by 
Strasburger ( 18 ) in the cases of Knautia and Althaea , and by Tischler (21) 
for Mirahilis Jalapa. 
It is, of course, possible that the layer containing the rodlets and 
spines may only become differentiated later, after the thickening bands have 
already been deposited, but I think this is unlikely both from the appear- 
ances in the present case and from analogy with what occurs in other plants. 
The thickening bands quickly increase in both thickness and breadth 
as fresh material is added to them by the protoplast. During the early 
stages the substance of the thickening bands appears to be soft and mucila- 
ginous ; their outer margin is ill defined and encroaches upon the clear 
spaces or layers referred to above, so that these become difficult to distin- 
guish, and the spine rudiments have the appearance of being embedded 
in the substance of the bands (Fig. 7). 
The external surface of the pollen-grain still remains smooth as the 
spine rudiments do not yet project above its surface. 
It may be noted that during the time that the various layers of the 
special wall and of the pollen-wall are being laid down by the protoplast, 
kinoplasmic fibres can clearly be distinguished running between the nuclear 
membranes and the ‘ Hautschicht ’ at the periphery of the protoplast (Figs. 4 
6 a and 7). These fibrils can be traced back to the kinoplasmic radiations 
which surround the nuclei during the telophase of the second meiotic 
division. The persistence of fibrillar differentiations of this kind is by 
no means uncommon during the earlier stages of the development of the 
pollen-grain, and I have met with it in several other plants besides Ipomoea. 
It seems quite probable that influences of some kind are distributed 
along these fibrils from the nucleus to the ‘ Hautschicht’ which is taking an 
active part in the formation of the new cell-wall lamellae. 
Up to the present the young pollen-grains have remained enclosed 
within the special wall. Now, however, these walls break down into a 
diffuse, flocculent material which fills the cavity of the anther loculus. The 
triradiate middle lamellae of the special walls, which we previously recog- 
nized as granular lines in the middle of the callose-pectose walls, often 
remain intact for some time longer, and can be seen lying in the midst 
of the flocculent material derived from the degeneration of the rest of these 
walls (Fig. 3). 
Soon after this time the spine rudiments have grown sufficiently centri- 
