736 PART IV. THE PHYSIOLOGY OF PLANTS. 



root-hairs of Trianea bogotensis, hairs of the stamens of Trades- 

 cantia, etc.). The movement takes place in the more fluid por- 

 tion of the protoplasm, and is made evident by the granules of 

 various kinds which are carried along by the currrent. The 

 direction of the movement varies somewhat according to circum- 

 stances : the current travels in one direction, and this simple 

 longitudinal movement is all that can be observed in plasmodia 

 and in hyphae ; but in cells, owing to their shortness, it can be 

 observed to travel up one long side, across the end, and down the 

 other side; and when the cytoplasm forms not merely a parietal 

 layer, but has strands traversing the vacuole (e.g. Fig. 46 D), 

 currents can be observed in these strands also. 



The contractile vacuoles (see p. 102) are small, more or less 

 nearly spherical, cavities which make their appearance in the 

 protoplasm and then suddenly disappear. In their relatively 

 slow expansion (diastole), they become filled with cell-sa,p, which 

 is forced out on the sudden contraction (systole). They have 

 been exclusively found in motile organisms, such as Volvox, 

 Gonium, Eudorina, the plasmodia of Myxomycetes, the zoospores 

 of many Algae and of some Fungi. 



In the second place the protoplasmic movements which involve 

 locomotion have to be considered. The simplest case of this is the 

 amoeboid movement exhibited, among plants, by the zoospores of 

 the Myxomycetes and of some Algae, and by the naked masses of 

 protoplasm which constitute the plasmodia of the Myxomycetes. 

 There is here no specialised motile organ, but any part of the 

 protoplasm may be protruded as a pseudopodium into which the 

 remainder of the protoplasm gradually flows, and thus locomotion 

 of the whole is effected. 



The locomotory movements of most zoospores, of spermatozoids, 

 and of entire organisms such as Volvox, Pandorina, etc., among 

 Algae, is effected by means of specialised motile organs, which are 

 delicate protoplasmic filaments termed cilia (see p. 102) ; each 

 cell may have one, two, four, or many cilia (see Figs. 168, 177, 

 181, etc.). 



Locomotion is also exhibited by other Algae, such as Diatoms, Oscillatorias, 

 etc., as also by some Sohizomycetes, but the mechanism is not fully under- 

 stood. 



B. Movements of Cellular Members. Instances of the move- 

 ment of parts of plants consisting of one or more cells having a 

 cell-wall, are afforded by all growing members, and by some 



