PROTOPLASM AND NUCLEUS. 



43 



and does not, when the cell distends, contract to a mere thin membrane. If the whole 

 mass of protoplasm 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 (Nagcli). 

 Examples occur in Characeae, in many other submerged water-plants, as Vallisneria, 

 Ceratophyllum, Hydrilleae, root-hairs of Hydrocharis ; the globular nucleus, when present 

 (in Characeae it soon disappears), is carried along with the current. The protoplasmic 

 body which encloses a large sap-cavity may, however, possess ridge-like prominences 

 arranged in a net-work, the substance of w^hich flows in different directions ; by this 

 means the nucleus may either, relatively, remain at rest, and, in a certain manner, form 

 the centre of movement, cr it is carried along with it. Cases of this kind occur tolerably 



Fig. 44.— .7 stellate hair on the calyx of the young flower-bud o( Aithcen rosea; thicker portions of protoplasm lie on the proto- 

 plasm-sac of each cell ; these are in the act of 'streaming' motion (indicated by the arrows). B epidermis (fj>) with the basal portion 

 of a mature stellate hair, showing the structure of the wall (X550). 



frequently in the hairs of land-plants (as in the stinging hairs oiUrtica urens, sieWcXte hairs 

 of Altbaa rosea). But the strings of protoplasm which show these currents may also 

 penetrate the sap-cavity of the cell; not unfrequently {e.g. Spirogyra, hairs of Cucurbita) 

 the nucleus then lies in its centre, enveloped by a mass of protoplasm ; the strings unite 

 it with the protoplasm-sac w^hich clothes the cell-wall. These strings or threads, stretch- 

 ing across the sap-current, may at first arise from the thin lamellae of protoplasm 

 which in younger quickly growing cells still separate adjoining vacuoli; w^hen these 

 finally flow together into a single sap-cavity, the thicker parts of these lamellae (Fig. i, B) 

 may remain as strings, forming a more or less irregular net-work, which at first cor- 

 responds, in posit:on and size, to the vacuoli that have now coalesced, but subse- 

 quently, as the cell continues to grow, and in consequence of the internal movements 



