209 
Beer. — Studies in Spore Development. 
prongs have an hemispherical outline in transverse section. A stainable, 
homogeneous material lies between the rodlets under the reticulate outer 
lamella. 
In sections which have been mounted in a drop of glycerine containing 
a little methylene blue and fuchsin mixture an interesting differentiation of 
the parts of the pollen-wall can be seen. The intine stains light red, the 
‘ mesospore ’ is blue, the reticulate outer lamella of the exine, as well as 
the spines and the rodlets, is green, whilst the homogeneous sub-reticulate 
substance (between the rodlets) colours deeply red. 
The mature spines of the exine measure between 12 and 14 /x in length ; 
they are usually fusiform in outline, although I have occasionally found 
them with a dichotomously branched apex (Fig. 19). 
The rodlets vary a good deal in size ; their shape is usually like that of 
a drumstick with a part of the knob or head just projecting through the 
perforated outer lamella of the exine (Fig. 17). 
The tapetal cells do not break down and scatter their contents between 
the pollen-grains, but they retain their membranes intact until the last. 
This tapetum, therefore, belongs to the ‘ secretion-tapeta ’ of Goebel. 
Deeply staining fibres and granules occur in the cytoplasm of the 
tapetal cells of Ipomoea during the middle period of anther development. 
These are most probably similar to the chromidial structures which have 
been described in the tapeta of several other plants {Nymphaea alba , Oenothera , 
Ribes, L ilium Martagon , Iris germanica, Syringa chinensis). I have not 
succeeded in tracing their origin in Ipomoea , but these structures are fre- 
quently aggregated in the neighbourhood of a nucleus in a manner which 
suggests their origin from this body (Fig. 20). Two nuclei most often occur 
in each tapetal cell during the development of the pollen-grains, and these 
may still be seen as somewhat shrunken, degenerating bodies in stamens 
which are nearly mature.. 
From the foregoing account of the development of these pollen-grains 
it will have been seen that there is no contraction of the protoplast from the 
pollen-wall at any time, even though the cytoplasm of the pollen-grain is at 
one stage represented only by a thin, hollow shell of material. 
Nevertheless, it is noteworthy that practically the entire growth of the 
spines and the rodlets takes place after the rudiments of these structures 
have been separated from direct contact with the protoplasm by the inter- 
polation of the thickening bands of the exine (‘ mesospore ’). That the growth 
of these structures is considerable will be seen from the fact that the spines 
increase in length from a rudiment which is too minute for measurement, to 
a comparatively massive spine with a length of 12 to 14 /x in the mature 
pollen-grain. It appears to me, therefore, that the growth of these spines 
and rodlets, which are in contact neither with the pollen-protoplast nor with 
P 
