PROTOPLASMIC MOVEMENTS. 49 



make some parts of the section useless, but others will always be 

 found which admit of undisturbed observation. The streaming is 

 not noticeable in the cells of the uninjured plant, but is induced 

 by the stimulus of an injury, and therefore by the preparation of 

 the section. This does not happen immediately, and it may be 

 necessary to wait some little time before strong streaming is visible ; 

 it can be best followed in the wide elongated cells which form the 

 internal tissue of the leaf. At a low room-temperature the move- 

 ment is sluggish, but it can be hastened by slight warming of the 

 microscope slide, as, for example, by laying it for a minute or so 

 on the hand. The stream circles around the entire cell, without, 

 in most cases, to any extent deviating from its direction parallel 

 to the long axis. The "neutral band" is fairly broad, and it is 

 easy in this case to see that the nearer the moving cytoplasm is 

 to the neutral band the slower is its movement, and vice versa. 

 The stream carries with it green coloured chlorophyll grains and 

 the nucleus. The latter is flattened into the form of a disk. 

 From time to time it comes into sight, but as a rule it is con- 

 cealed by chlorophyll grains. Not infrequently it sticks at a 

 turning point, then the accompanying chlorophyll grains also 

 halt with it, till, an instant later, all again are drawn into the 

 stream. The direction of the streaming changes from cell to cell 

 without any regularity. If dilute glycerine or 15 or 20 per cent. 

 sugar solution is permitted to act upon the section, the proto- 

 plasmic sac can be seen to withdraw from the cell-wall, and the 

 continuance of the streaming at the first moment of contraction 

 can be readily made out, while it is often continued when the 

 cytoplasm has collected into a rounded ball. 



Movements in Elodea. Vallisnena may also be replaced, with 

 approximately like results, by the widely-spread river and pond 

 weed, Elodea canadensis, the leaves of which are so transparent 

 that they can be used without preparation. The leaf to be 

 studied must, however, be severed from the stem which bears 

 it, for here, as in Vallisneria, strong movements are induced by 

 injury. As in Vallisneria the cells near the mid-rib are elongated 

 and narrow ; they show simple rotation, made evident by the 

 chlorophyll bodies which are hurried around in the current. The 

 shorter and broader cells of the general lamina of Elodea may 

 also show us a simple kind of circulation, the vacuole, as in the 

 staminal hairs of Tradescantia, being traversed by cytoplasmic 

 threads (often few and coarse) which suspend the nucleus in the 



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