PROTOPLASM AND NUCLEUS. 
43 
this condition is permanent. If trie whole protoplasm-mass withdraws to the cell- 
wall, enclosing a single large vacuole (the Sap-ca'vity 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 (Nägeli). 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 Gharacese 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 {e.g. 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-cavity, the thicker parts of the 
lamellae (Fig. i, 5, p. 2) remain as threads, forming a more or less irregular net- work, 
which at first corresponds to the coalescent vacuoles, but which undergoes further 
changes of form as the cell continues to grow, and in consequence of the internal 
movements of the whole protoplasm. New threads also make their appearance ; ridge- 
like portions arise from the peripheral protoplasm, or even on the thicker threads, and 
