by cold, like the circulation of the blood in animals, because the activity of the vital energy bears a precise relation to the quantity of heat received. The gyration is instantly arrested by a shock of electricity. The reproductive organs of the Characeæ are of two kinds, both growing in the axils of the branchlets, namely, dark-red globules, which are antheridia, and nucules or pistillidia, which contain germ cells. Sometimes they are found in different individuals, but in most of the Nitellas they are in the same individual, the globules being placed closely below the nucules, as in fig. 41, A, B. The envelope of the nearly spherical globules is formed of eight spherico-triangular valves. From the middle of the interior surface of these valves, a perpendicular orange-coloured column extends to the centre of the globule, where its summit is crowned with a mass of confervoid filaments, which are formed of a linear succession of minute cells; while from the base of the column, bands of orange-coloured spherules imbedded in gelatine radiate along the interior surface of the valve to its margin as shown at C, in the same figure. After successive changes in the matter within the confervoid filaments, (fig. 42, D-G), the microscope shows that ‘in every one of the cells there is formed a spiral thread of two or three coils, which, at first motionless, after a time begins to move and revolve within the cell; at last the cell wall gives way, and the spiral thread makes its way out, partially lengthens itself, and moves actively through the water in a tolerably determinate direction, by the lashing action of two long and very delicate filaments with which it is furnished’[65] (fig. 42 H). [Illustration: Fig. 41. Antheridia of Chara fragilis:—A, antheridium developed at base of nucule; B, do., the nucule enlarged, and the antheridium laid open by the separation of its valves; C, one of the valves, with its group of antheridial filaments.] The nucule is an ovoid sac with five long cells spirally twisted round it, the sac being full of a viscous fluid containing globules of starch and oil. This nucule falls off when fertilized by the spirally-coiled ciliated bodies, and then germinates. The Characeæ may also be artificially reproduced by cuttings; while under favourable circumstances they are reproduced by nodular bodies rich in starch grains, which arise from the creeping root, and also by clusters of cells called bulbils filled with starch grains, which arise from a modification of the nodes.