ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
207 
only the development and the escape of the zoospores which are promoted 
by the saline solution, their first formation being the result of the 
internal nature of the cell. Some organic substances, as maltose and 
dulcite, also promote the formation of zoospores. 
The tendency to produce gametes is not so easily excited, but can be 
brought about by cultivation in a 7-10 per cent, solution of cane-sugar, 
the presence of nutrient salts being excluded ; and this can take place 
even in the dark. 
Further experiments showed that, in a single net consisting of 
equivalent sister-cells, some of the cells can be excited, by external 
conditions, to develope zoospores, others to develope gametes. In a net 
which is commencing to produce gametes, a change to the formation of 
zoospores can be brought about by immersion in the above-named saline 
nutrient solution ; a change from the sexual to the non-sexual condition 
is not so easily effected, but can be brought about by cultivating in 
maltose or dulcite. 
The general conclusion of the author is that there is not in Hydro- 
dictyon any true and necessary alternation of sexual and non-sexual 
generations such as is displayed in the Muscineae and Vascular Crypto- 
gams, but that every cell of the net has the capacity for producing both 
kinds of organ, and that it depends on external conditions which of the 
two forms of reproductive organ is brought into existence; favourable 
conditions tending, as a rule, to the production of non-sexual, unfavourable 
conditions to the production of sexual organs. 
Nursing of the Embryo.* — Mr. T. Johnson describes the peculiar 
mode of growth of the embryo in the parasitic Myzodendron punctulatum, 
belonging to the Loranthaceae. After fertilization the secondary nucleus 
of the embryo-sac divides repeatedly into a row of nuclei extending the 
whole length of the embryo-sac, which are soon separated by cell- walls, 
so that the interior of the embryo-sac is occupied by a uniserial column 
of endosperm-cells. During this time the narrow antipodal end of the 
embryo-sac has elongated upwards and backwards in the body of the 
placenta. It then makes a sharp bend upon itself, and continues its 
penetrating course, in a more or less winding manner, through the free 
column of the placenta, and on through the tract of tissue continuous 
with this, until it reaches the base of the flower, where its tip dilates and 
becomes imbedded in the vascular cup formed by the three carpellary 
vascular bundles, between the tip of which and the descending tip of 
the embryo-sac a few layers of rich parenchymatous cells intervene. 
Throughout its prolongation the embryo-sac remains a uniserial 
column of uninucleate richly protoplasmic cells. During the same 
time the nucellus-portion of the embryo-sac has become filled with 
endosperm-cells. The embryo, although divided into a small number 
of cells, remains for a long time undifferentiated, as in many other 
parasites. The main function of the embryo-sac tube is clearly nutri- 
tive. The placenta being destitute of vascular bundles, it acts as a 
carrier of food from the floral vascular bundle to the developing seed. 
In the ripe seed it is still an open tube, though its protoplasm is reduced 
to a thin layer inclosing a large quantity of cell-sap. 
* Arm. of Bot., iii. (1889) pp. 179-206 (2 pis.). 
