ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
183 
represented to-day most abundantly by the Marattia forms and their 
allies, and was the most common Palaeozoic type of Filicales. From it 
the Gymnosperm lines, at least the Cycads and Conifers, were derived. 
The frequent independent appearance of heterospory probably results 
from inequalities of nutrition in connection w r ith the development of 
antherids and archegones. The retention of the megaspore, resulting in 
the seed habit, follows the extreme sterilisation of the megasporange 
which is attained with the organisation of but one megaspore. This was 
followed by the development of seed-coats, by immediate germination of 
the “ oospore,” and by the mature seed-structures. The first retained 
megaspores were doubtless directly exposed to the microspores ; and in 
Cordaites and Cycads a pollen-chamber of varying depth and extent is 
associated with the early stages of siphonogamy, with which spermato- 
zoid habit was more or less associated. The pollination of Gymnosperms 
is but a continuation of the ordinary method of dispersing aerial spores 
employed by Cryptogams ; the chief result of the retention of the 
megaspore upon the male gametophyte being the development of siphono- 
gamy. 
Physiological Effect of Plasmolysing Agents.* — From experiments 
made by M. R. H. True, it results that plants may be fatally affected, 
both by solutions acting osmotically and by solutions acting through 
their chemical properties. While many plants suffer when removed 
suddenly from salt water to fresh, and appear to be dependent on the 
presence of sodium chloride in the medium, the author states that both 
sodium chloride and potassium nitrate act as poisons to Spirogyra , 
operating through their chemical properties. 
Diseases of Plants.f — The experiments recorded by M. E. Laurent 
are extremely interesting from a practical as well as a pathological 
standpoint. The observations were made on various kinds of plants, 
chiefly potatoes, cultivated on a good soil. The ground was parcelled 
out into four lots, each lot being treated with a different kind of manure, 
e.g. sulphate of ammonia, kainite and potash, superphosphate of lime 
and phosphoric acid, lime. The effect of the following parasitic organ- 
isms, B. coli communis , B. Jiuorescens putidus, Phytophthora infestans, and 
Sclerotinia Libertiana , is discussed at some length. The conclusions 
arrived at are that B. coli communis is a saprophyte, but may become 
parasitic and acquire virulent properties by culture on alkalised potato, 
and by subsequent transference to the same kind of tuber. After culti- 
vating on other species, the pathogenic properties disappear. Hence 
diminished resistance on the part of the host seems to be the starting 
point from whence the transformation of saprophyte into parasite takes 
place. The different effect of the same manure on different plants, e.g. 
phosphate of potash, is explained by the fact that while some microbes 
require an alkaline medium, to others an acid is more suitable in order to 
facilitate the action of the microbic diastase, which, by dissolving the in- 
tervening vegetable cells, is to prepare the way for the subsequent action 
of the parasite. Hence the nature of the soil and the composition of the 
manure greatly influence the resistance of plants to their parasites ; and 
* Bot. Gazette, xxvi. (1898) pp. 407-16. 
f Ann. Inst. Pasteur, xiii. (1899) pp. 1-48. 
