THE PLANT-CELL 41 



the long axis of the cell. The effect of the rotating mass is that of 

 a broad stream running up one side of the cell and down the other. 



The second type of movement streaming or circulation is 

 much commoner, and may readily be seen in the cells of many hairs, 

 such as those of Geranium or Petunia. The large bristly hairs on 

 species of Cucurbita are especially good objects for demonstration, as 

 are the well-known stamen hairs of species of Tradescantia. In such 

 cells the nucleus is usually imbedded in a mass of cytoplasm from 

 which delicate threads or lamellae run to the peripheral cytoplasm 

 which surrounds the large sap-cavity. These radiating threads con- 

 sist of a sheath of hyaloplasm within which the granular plasma is 

 seen to be in active streaming motion. Similar movements may be 

 seen in the peripheral cytoplasm. The movements are for the 

 most part to and from the nucleus, and even in very delicate threads 

 two currents moving in opposite directions may often be noted, and 

 a stream which has been moving in one direction may have its 

 motion reversed. The mechanism governing these movements is 

 not clearly understood. 



Movements of Orientation Slow movements within the cell, result- 

 ing in the change in position of its organs, are not uncommon, and 

 can often be explained as a response to certain stimuli. The most 

 familiar of these movements is the change in position of the chro- 

 matophores under the influence of light. Similar movements of the 

 whole cell are seen in the free-swimming green zoospores of many 

 Algae, whose movements are strongly influenced by light. A good 

 example of the shifting of the chloroplasts within the cell is offered 

 by the Alga Mesocarpus (Fig. 83), where the single axile flat chro- 

 rnatophore revolves on its axis, presenting either the edge only, or 

 the whole surface, as the intensity of the* light varies. So in the 

 cells of a Moss leaf, the chromatophores spread themselves evenly 

 over the outer cell-wall if the light is diffuse, but retreat to the 

 lateral cell-walls and present their edges to the light if it is too 

 intense. These movements are obviously closely associated with 

 the question of the regulation of the intensity of light to which the 

 chromatophore is exposed. 



Water in Protoplasm All protoplasmic movements require the 

 presence of water, whether these are ciliary or amoeboid movements 

 of a naked protoplast, or movements within the protoplast. With- 

 out water the labile character of the protoplasm must cease, and 

 when it is withdrawn the protoplasm loses its viscid consistence, 

 and becomes hard and rigid. The withdrawal of water does not 

 necessarily kill the protoplasm, which may be restored to activity 

 by supplying water, but its activity is effectively checked. This is 

 illustrated in dried spores and seeds, which begin to grow as soon as 

 water is supplied. 



