LESSON 27.] CIRCULATION IN CELLS. 167 



tact with the earth and air on \vhich they feed, the latier and the 

 most important of these everywhere just the same, have no need 

 of locomotion, and so are generally fixed last to the spot where 

 they grow. 



481. Yet many plants move their parts freely, sometimes when 

 there is no occasion for it that we can understand, and sometimes 

 accomplishing by it some useful end. The sudden closing of the 

 leaflets of the Sensitive Plant, and the dropping of its leafstalk, 

 when jarred, also the sudden starting forwards of the stamens of the 

 Barberry at the touch, are familiar examples. Such cases seem at 

 first view so strange, and so d : fferent from what we expect of a plant, 

 that these plants are generally imagined to be endowed with a pe- 

 culiar faculty, denied to common vegetables. But a closer exam- 

 ination will show that plants generally share in this faculty ; that 

 similar movements may be detected in them all, only like those 

 of the hands of a clock, or of the shadow of a sun-dial they are 

 too slow for the motion to be directly seen. 



482. It is perfectly evident, also, that growth requires motion ; 

 that there is always an internal activity in living plants as well as 

 in animals, a power exerted which causes their fluids to move or 

 circulate, and carries materials from one part to another. Some 

 movements are mechanical ; but even these are generally directed 

 or controlled by the plant. Others must be as truly self-caused as 

 those of animals are. Let us glance at some of the principal sorts, 

 and see what light they throw upon vegetable life. 



483. Circulation ill Cells, From what we know of the anatomy of 

 plants, it is clear that they have no general circulation (like that of 

 all animals except the lowest), through a system of vessels opening 

 into each other (402, 410). But in plants each living cell carries 

 on a circulation of its own, at least when young and active. This 

 may be beautifully seen in the transparent stems of Chara and many 

 other water-plants, and in the leaves of the Fresh-water Tape-Grass 

 (Vallisneria), under a good microscope. Here the sap circulates, 

 often quite briskly in appearance, (but the motion is magnified as 

 well as the objects,) in a steady stream, just beneath the wall, 

 around each cell, passing up one side, across the end, down the 

 other, and so round to complete the circuit, carrying with it small 

 particles, or the larger green grains, which make the current more 

 visible. This circulation may also be observed in hairs, particularly 

 those on flowers, such as the jointed hairs of Spiderwort, looking 



