210 Beer . — Studies in Spore Development . 
the tapetal cytoplasm, is of quite the same character as the growth of the 
entire membranes of Isoetes , Selaginella , Oenothera , or Mirabilis . The 
present instance may not at once appear so striking as these latter cases are, 
but it is no less an interesting and clear example of the growth of a portion 
of the cell-membrane in entire independence of the direct influence of the 
living protoplasm. 
In the case of the spines and rodlets of Ipomoea , as in that of the mem- 
branes mentioned above, the origin and first differentiation takes place under 
the direct control of the protoplasm, but, once formed, the further growth 
may continue and, moreover, maintain throughout the characteristic shape 
and structure of the part, quite independent of any immediate guidance 
from the living protoplast, provided only the material necessary for this 
growth is forthcoming. 
In conclusion, I desire to express my indebtedness to the Government 
Grant Committee of the Royal Society for the loan of a Zeiss Y2-^ nc ^ 
apochromatic objective (1*40 aperture), which has been invaluable through- 
out this research. 
Summary. 
1. At the conclusion of the second meiotic division the chromosomes 
remain distinguishable for a short time after the reconstruction of the 
daughter nuclei, but subsequently their substance becomes completely 
dispersed over the linin-reticulum. 
Chromatic aggregations also occur in many of the nuclei of the anther 
tissues, notably in those of the young vascular bundle, but the size and 
number of these aggregations are quite inconstant. 
2. The pollen-wall, when it first becomes recognizable, is a single, 
delicate membrane in which no structure can be distinguished. 
3. The exine of slightly older pollen-grains consists of an outer 
lamella, upon the inner face of which is deposited a network of thickening 
bands. At the angles of the meshes of this reticulum the rudiments of the 
future spines already occur. Between the thickening bands and the outer 
lamella a narrow unstained space or layer can be seen ; this marks the 
position in which the rodlets of the older pollen-grains are developed. 
4. The outer surface of the pollen-grain is at first quite smooth. The 
spine rudiments appear to project towards the pollen-cavity, so that they 
push the thickening bands inwards at these points into a series of short, 
internal spinous structures, but they do not extend beyond the outer surface 
of the grain. 
5. During the earlier stages of development, whilst the layers of the 
special wall and the pollen-wall are being initiated, kinoplasmic fibrils 
connect the nuclear membrane with the £ Hautschicht ’ of the pollen-proto- 
plast. Influences of some kind are probably passing along these fibrils 
