CLEAVAGE IN DIDYMIUM MELANOSPERMUM (PERS.) MACBR. 1 33 
the protoplasm to shrink away from them in fixation. At their inner 
and outer ends the threads swell slightly and become hollow where they 
join the peridium and columella respectively. Throughout the rest 
of their extent they are solid and show no surface sculpturing nor 
markings of any kind. 
At this stage the nuclei are scattered rather uniformly through the 
protoplasmic mass (figs. I and 2). With the magnification used for 
figure 2 the nuclei appear as hardly more than dots and those less deeply 
stained are scarcely visible. In some cases there is a tendency for the 
nuclei to be surrounded with a vacuole. This, however, is true of 
relatively few of the nuclei in any particular section, while the re- 
mainder show themselves in close contact with the cytoplasm over their 
whole surface. Such cases can hardly be due to shrinkage in fixation, 
since they are scattered among other nuclei which show no such 
peculiarity. Nuclei enclosed in vacuoles have been observed in the 
root hairs of Chara and Debski (10) has shown that in this case the 
vacuoles do arise during fixation. 
The nuclei at this stage are of very unequal size. The smallest 
nuclei are extremely dense chromatic masses. In some of these it is 
almost impossible to make out a nucleole; in others, however, and 
especially in those that are a little larger, by the use of the triple stain 
a red nucleole can be conspicuously differentiated from blue chromatin 
as in figure 7. I have not found any nuclear division figures in my 
sections at this earliest stage, though they are frequent at a very little 
later stage. We may distinguish three distinct types of nuclei at this 
stage on the basis of their size and appearance. The type of smallest 
size is that just described. Those of middle size are abundant and 
very typically differentiated. The nucleole is sharply defined from 
the chromatin, is globular in form and dense but transparent and 
stains bright red. The chromatin consists of strands and granules 
forming the ordinary netted or reticulated figure of resting nuclei. 
The nuclear membrane is sharply defined and the nuclear cavity 
contains a considerable amount of clear cell sap in the meshes of the 
chromatin network. Nuclei of the third type are distinctly larger, 
and show very different staining qualities than the smaller nuclei. 
Their general consistency is very like that of the cytoplasm and they 
do not appear nearly as conspicuous as do the other two types. Their 
nuclear membranes are, however, very sharply defined. Their nucle- 
oles also are bright red stained globules no larger than the nucleoles of 
