I 



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-cwvity 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 diff"erent directions; the nucleus may then either remain at rest and form 

 the centre of movement, or be carried along with the current. Cases 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 (ej>) 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, B, 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 



