560 Saxton.—Contributions to the Life-History of Callitris. 
thallus of Callitris has not been seen, but doubtless takes place in the 
normal manner. 
The archegonia are formed in a single group abutting on the side of 
the pollen-tube (Fig. 12), except where two pollen-tubes are present, when 
two such groups are organized. No other archegonia, not in contact with 
the pollen-tube, are found. The number of archegonia in a group is 
usually about seventeen to twenty. The archegonia are considerably 
smaller than those of Widdringtonia , but probably their development is the 
same. The neck soon disappears, and the adjacent part of the pollen-tube 
breaks down, so that the contents of the archegonium are freely exposed 
to those of the tube (Fig. 13). The oosphere nucleus has one, or some¬ 
times two, large hollow nucleoli, and is situated near the base of the arche¬ 
gonium, no vacuole being present as a rule. Occasionally an aster-like 
structure is evident near the top of the archegonium (Fig. 13). This may 
possibly represent the remains of the ventral canal nucleus. 
5. The Male Gametophyte. Some of the microspore mother-cells, 
probably shortly before division, are shown in Fig. 14. Unfortunately 
stages to illustrate microsporogenesis have been missed. The wall of the 
mother-cell is rather thick, and between this and the central nucleus is 
a space packed with starch grains (Fig. 14). The wall of the mature pollen 
grain is, however, much thicker than that of the mother-cell. As far as has 
been seen, the microspore nucleus does not divide before the spores are shed ; 
this point has been investigated in C. verrucosa and C. robusta. It is not 
very easy to make out the structure of the mature grain owing to the thick 
hard wall, which is difficult to penetrate and to cut, but in the thousands 
of spores examined not one has appeared to contain a second nucleus. 
This does not agree with the statement made by Coker ( 3 ) that two 
nuclei are present at the time of shedding in a species of Callitris , but no 
statement is made by him as to which species was the subject of investigation. 
I have found it impossible at present to follow the early development 
of the pollen-tube. The nucellar beak seems to offer great resistance to the 
microtome knife, and it is comparatively seldom that a clear view even of 
the tube itself is obtained, much less of the contents. Later stages, after the 
cutting-off of the two sterile nuclei, are quite easy to follow. These two 
nuclei are invariably equal in size and staining-capacity and are usually 
abreast of one another in the tube (Fig. 15), so that it is impossible to 
distinguish between ‘ tube * and ‘ stalk ’ nuclei. They are always to be seen 
in the tube and do not tend to disappear as they do in Widdringtonia. When 
the male cells are formed, starch grains are found in the tube, contrary to 
what is the case in Widdringtonia. 
The two male cells have very dense cytoplasm with a distinct limiting 
membrane or wall and a large central nucleus, with a single large, hollow or 
reticulate nucleolus (Fig. 16). In this figure the two sterile nuclei are quite 
