CIRCULATION OF PROTOPLASM. 29 



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, some of the best examples 

 being certain leaf-cells of Vallisneria and 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 travel snot only in 

 a peripheral stream but also in strands which run across through 

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

 Circulation is well seen in cells composing the hairs of various 



FIG. 17. Flower-cluster () and single stamen (6) of a cultivated spiderwort ( Tradescantia) ; 

 /i, hairs upon the stamen, a, slightly reduced; 6, slightly enlarged. 



plants, such as the common nettle ( Urtica), the spiderwort 

 (Tradescantia). the hollyhock (Althcea\ and certain species of 

 gourds (Cucurbitcb). It may be conveniently studied in the 

 hairs upon the stamens of the cultivated spiderwort (Trades- 

 cantia). The flower of this plant is shown in Fig. 17 and one 

 of the stamens with its hairs at &. Each hair consists of a single 

 row of elongated cells covered by delicate membranes and con- 

 nected by their ends. As in Nitella, the protoplasm does not 

 till the cavity of the sac, but forms a thin lining (primordial 

 utricle) on its inner face (Fig. 18). From this layer delicate 

 threads of protoplasm reach into and pass through the central 

 cavity, where they often branch and are connected together so as 

 to form a verv loose network. The nucleus (n} is embedded 



