626 
Kershaw . — Structure and Development of the 
to attempt at some future date. The present paper will be mainly descriptive, 
with an indication of the direction in which the facts for Bowenia would 
seem to point in a discussion of the general phylogenetic problem. 
Description of Ovules. 
The megasporophylls form a compact strobilus and have a well-defined 
stalk which expands into a peltate head with a flat or slightly ridged apex 
(Fig. 9, PI. LXI). The peltate head is elongated horizontally, and an 
ovule is borne on the under side of each of the horizontal extensions. The 
general features of the series of ovules to be described are shown in Text- 
figs. 1-4 and 14-16, which are outline camera lucida drawings. The main 
features of the ovules of various ages will be described in order. 
( a ) Unpollinated Ovules. 
The youngest ovule obtained (Text-fig. 1) appears as a small swelling 
of the under side of the sporophyll, with very little constriction at the point 
of attachment, for no distinct stalk is yet differentiated. 
The nucellus is surrounded by the single, differentiated integument 
a division line being formed by rows of flattened cells which extend from 
the base of the ovule to the epidermis. A slight cleft in this region of the 
epidermis indicates the beginning of growth of the free parts of nucellus and 
integument. 1 The tissue of the nucellus consists of longitudinal rows of 
cells traceable to the flat apex and evidently produced by periclinal divisions 
of the epidermis. In the centre of the nucellar tissue there is a rounded 
mass of cells with rather denser contents than the surrounding cells. Further 
growth is indicated in this tissue, for many of the cells show signs of a coming 
division. This is the sporogenous tissue which later becomes more con- 
spicuous and, by analogy with other Cycads 2 in which the earlier develop- 
ment has been worked out, may have formed by the division of the original 
archesporial cell. 
Situated usually slightly above the centre in the sporogenous tissue is 
a conspicuous cell, 0*04 mm. long, larger and with a rather thicker wall than 
the surrounding cells (Fig. 1, PI. LXI). This is the mother-cell of the 
embryo-sac. The protoplasm of the cell is unvacuolated and the large 
central nucleus has a distinct nucleolus. Ovules of approximately the same 
age show a rather later condition of the nucleus of the megaspore mother- 
cell (Fig. 2, PI. LXI). This seems very like the synapsis condition in 
preparation for the coming reduction division. The chromatin matter is 
contracted in a mass to one side and the rest of the enlarged nuclear area is 
clear. Since, however, the material was only roughly fixed in 70 per cent, 
spirit it was not suitable for cytological work. The vascular bundles which 
1 Cf. Lang, loc. cit., Fig. 2, PI. XVII. 
2 F. G. Smith : Development of Strobilus of Zaniia. Bot. Gaz., vol. 1, 1910. 
