. Text-book of botany, morphological and physiological. Botany. PROTOPLASM AND NUCLEUS. 43 this condition is permanent. If the whole protoplasm-mass withdraws to the cell- wall, enclosing a single large vacuole (the Sap-cavity of the cell), all the particles of protoplasm, flowing in one direction, may form a continuous broad current encircling the cell (rotation), the direction of which is always such as to describe the longest course round the cell-cavity (Nageli). Examples occur in Characeae, and in many other submerged water-plants, as Fallisneria, Ceratophyllum, Hydrilla, and root-hairs o
. Text-book of botany, morphological and physiological. Botany. PROTOPLASM AND NUCLEUS. 43 this condition is permanent. If the whole protoplasm-mass withdraws to the cell- wall, enclosing a single large vacuole (the Sap-cavity of the cell), all the particles of protoplasm, flowing in one direction, may form a continuous broad current encircling the cell (rotation), the direction of which is always such as to describe the longest course round the cell-cavity (Nageli). Examples occur in Characeae, and in many other submerged water-plants, as Fallisneria, Ceratophyllum, Hydrilla, and root-hairs of Hydrocharis; the globular nucleus, when present (in Characeae it soon disappears), is carried along with the current. The protoplasm-mass which encloses a large sap-cavity may, however, possess a net-work of ridge-like prominences, the substance of which flows in different directions; the nucleus may then either remain at rest and form the centre of movement, or be carried along with the current. Gases of this kind occur tolerably frequently in the hairs of land-plants, as in the stinging hairs of the. Fig. 42.—^ stellate hair on the calyx of the young flower-bud of the hollyhock ; thicker ridges of protoplasm project into the sap-cavity of each cell; these are in 'streaming' motion (indicated by the arrows). B epidermis {ep) with the basal portion of a mature stellate hair, showing the structure of the wall {X 550). stinging-nettle, and the stellate hairs of the hollyhock. But threads of protoplasm which exhibit these currents may also penetrate the sap-cavity of the cell; not un- frequently ( Spirogyra, hairs of Cucurbita) the nucleus then lies in the centre, enveloped by a mass of protoplasm, the threads uniting it with the layer which clothes the cell-wall. These threads, stretching across the sap-cavity, arise from the thin lamellae of protoplasm which in young quickly-growing cells separate adjoining vacuoles; when these finally flow together into a single sap-cav
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