PROTOPLASMIC MO VEMENTS. 



29 



protoplasm is surrounded by a delicate membrane which thus 

 forms a sac enclosing the protoplasm. In very young cells the 

 protoplasm entirely tills the sac ; but as the cell grows older a 

 drop of liquid appears near the centre of the mass and increases 

 in size until the protoplasm is reduced to a thin layer {jyinmor- 

 dial utricle)^ lining the inner surface of the memljrane (compare 

 Fig. 2). In favorable cases the entire mass of protoplasm is 

 seen to be flowing steadily around the inside of the sac, as in- 

 dicated by the arrows in Fig. IT. It moves upwards on one 

 side, downwards on the opposite side, and in 02)posite directions 

 across the ends, forming, an unbroken circuit. The flow is ren- 

 dered more conspicuous by various granules and other lifeless 

 masses floating in the protoplasm and by the large oval nucleus 

 or nuclei, all of which are swept onward by the current in its 

 ceaseless round. A similar rotation of protoplasm occurs in many 

 other vegetal cells, one of the best examples being the leaf-cells 

 of Anacharis. 



A second and somewhat more intricate kind of movement in 

 vegetal protoplasm is known as circulation. This differs from 

 rotation chiefly in the fact that the protoplasm travels not only in 

 a peripheral stream but also in strands which run across through 

 the central space (vacuole) and thus form a loose network. Cir- 



Fio. 18.— Flower-cluster {a) and single stamen (h) of a cultivated spidervrort {Trades' 

 cantia). 7i, hairs upon the stamen, a, slightly reduced ; b, slightly enlarged 



dilation is well seen in cells composing the hairs of various ])lants, 

 such as the common nettle (Urtica), the spiderwort (Trades- 



