ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
511 
about the end of April, at the expense of their subtending bract, and are 
fully developed by the July preceding their blossoming. The female 
flowers, on the other hand, make their appearance on twigs which are 
formed in the spring, and the whole of their development is accomplished 
in the same year. They are, therefore, fecundated by the pollen of male 
flowers formed at a very much earlier period. By the month of July 
the catkin is about a centimetre in length, and the pollen-sacs are then 
already formed, although the pollen-grains are not fully differentiated 
until the moment of flowering. 
Viviparous Grasses.* — According to Herr J. H. Wakker, the pheno- 
menon of viviparity may occur either in the upper part only of the spike, 
or in both the upper and the lower part. Viviparous growth appears to 
take place almost entirely in the autumn, and to be dependent on a 
large supply of animal manure to the soil, which promotes the develop- 
ment of the vegetative organs, while it is unfavourable to the formation 
of flowers. 
Activity of the Cambium in Trees.] - — With the view of determining 
the period of the annual reappearance and extinction of the activity of 
the cambium-ring, M. E. Mer has made a series of observations on a 
number of dicotyledonous and gymnospermous trees — the oak, beech, 
hornbeam, lime, poplar, fir, &c. He finds an obvious relation between 
the duration and the intensity of the activity of the cambium. It is in 
the regions of the trunk where the vegetative activity is most pronounced 
— whether because they are the younger or because they receive the most 
nutriment — that the cambium first wakes into activity, viz. in the ex- 
tremities of the branches, in the basal swellings of the boughs, in the 
lower part of the trunk of vigorous trees. Under all conditions, on the 
other hand, where vegetation is retarded, the activity of the cambium 
begins later and ceases earlier, as in the lower parts of the branches and 
of the trunk of trees which are massed together. In young trees, 
especially when they are crowded together, the annual rings of wood are 
often broader in the middle and upper than in the lower part of the 
trunk ; while in older isolated trees, especially during the period of 
most rapid growth, the breadth of the rings gradually increases from 
above downwards, showing a great activity of the generating layer in 
this latter region. 
Assimilation of Free Nitrogen by Plants.]: — Herr B. Frank recurs 
to the subject of the power of leguminous plants to absorb free nitrogen 
directly from the atmosphere, and details the results of a series of 
experiments, chiefly on Lupinus luteus and Pisum sativum. He found 
that, even when the presence of a symbiotic fungus is excluded, both 
these plants can obtain their nitrogen from nitrogenous manures, calcium 
nitrate, ammonium phosphate, or urea ; but that the symbiotic fungus 
alone, without these manures, is more favourable to the plant than the 
manures without the fungus. The pea is more dependent on the nitro- 
genous manure than is the lupin. In good soils both these plants have 
* SB. Niederl. Bot. Ver., Feb. 7, 1891. See Bot. Centralbl., xlix. (1892) p. 142. 
■f Comptes Kendus, cxiv. (1892) pp. 242-5. 
% Deutsche Landwirthsch. Presse, 1891, pp. 779-80. See Bot. Centralbl., 1892, 
Beih., p. 71. Cf. this Journal, ante, p. 234. 
