MOVEMENTS OF PROTOPLASM IN CELL-CAVITIES. 33 



at rest at any time, except perhaps during periods of drought in summer and of 

 frost in winter, and in seeds during their time of quiescence. This applies par- 

 ticularly to immature cells. In them the protoplast forms a solid body whose 

 substance entirely fills the cell-cavity. The young cell, however, grows up quickly, 

 its cavity is enlarged, and the space, hitherto filled by the protoplast, becomes two 

 or three times as large as before. But the increase of volume on the part of the 

 protoplast itself does not keep pace with the enlargement of its habitation. It is 

 true that it continues" to cling closely to the inner face of the cell- wall, thus forming 

 the primordial utricle; but the more central part of its body relaxes, and in it are 

 formed vacant spaces, the vacuoles above mentioned, wherein collects a watery 

 fluid known as the "cell-sap." The portions of protoplasm which lie between 

 the vacuoles resolve themselves gradually into thin partitions bounding them; and 

 lastly, these partitions split up into bands, bridles, and threads, which stretch across 

 the cell-cavity from one side of the primordial utricle to the other, and are woven 

 together here and there where they intersect. With these protoplasmic strands we 

 have already become acquainted. 



But the protoplasm in the interior of a growing cell, whilst relaxing and 

 breaking up, also becomes motile if the liquid attains a certain temperature, and 

 then the appearance presented is like that of a lump of wax melting under the 

 action of heat. These movements may be observed very clearly under the micro- 

 scope in the case of large cells with thin and very transparent cell-membranes, 

 especially when the colourless, translucent, and gelatinous substance of the proto- 

 plasm — not always sharply defined in contour — happens to be studded with 

 minute dark granules, the so-called "microsomata." These granules are driven 

 backwards and forwards with the stream, like particles of mud in turbid water, and 

 their motion reveals that of the protoplasm wherein they are embedded. Seeing 

 particles gliding in all directions through the cell-cavity, arranged irregularly in 

 chains, rows, and clusters in the protoplasmic strands, we are justified in concluding 

 that this motion takes place in the substance of the strands itself. The movement, 

 moreover, is not confined to isolated strands, but occurs in all. Granular currents 

 flow hither and thither, now uniting, now again dividing. They often run in 

 opposite directions even when only a trifling distance apart; sometimes two chains 

 are drifted in this way when actually close together in the same band of proto- 

 plasm. The streams pour along the primordial utricle and whilst there divide into 

 a number of arms, meeting and stemming one another and forming little eddies; 

 then they are gathered together again and turn into another strand of the more 

 central protoplasm. The individual granules in the currents are seen to move with 

 unequal rapidity according to their sizes; the smaller particles progress faster than 

 the larger, and the larger are often overtaken by the less, and when this happens 

 the result often is that the entire stream stops. If so, however, the crowded 

 particles are suddenly rolled forward again at a swifter pace, like bits of stone in 

 the bed of a river as it passes from a level valley into a gorge. The course of the 

 streaming protoplasm remains throughout sharply ma.rked off from the watery sap 



VOL.X 3 



