222 CIRCULATION OF FLUID WITHIN CELLS. 



some time afterwards, that it acquires any considerable firmness. 

 During the period of the formation of the cell, the space between 

 the membrane and the original disk is filled with fluid ; and in 

 this, a regular circulation may be seen to take place, several 

 currents proceeding from the nucleus (or cytoblast as it is techni- 

 cally called), and returning to it again. 



356, When the cell becomes mature, the original disk is usually 

 absorbed, and no further movement of fluid is seen within the 

 cavity ; but there are some cells in which it always remains, ap- 

 pearing as a dark spot in their walls ; and in these, the circulation 

 of fluid generally continues. This circulation may be well seen is 

 the beaded hairs of the Tradescantia Virgmica (Virginian Spider- 

 wort), which consists of several distinct cells ; at the bottom of 

 each of these, the disk or nucleus may be seen, and several currents 



may be observed to proceed from it and return to 

 it again. It is a circulation of this kind, which 

 has excited much attention in the stem and 

 branches of the Chara (a little cryptogamic aquatic 

 plant), which consist only of large cells laid end 

 to end. The fluid passes down one side of the 

 stem, and up the other, turning round at each 

 CURRENTS m THE extremity. If the stem (which is usually com- 

 HAIRS OF TRADKS- posed of a single cell, sometimes many inches 

 long) have a thread tied round its centre, so as to 

 separate the original cell into two, each of these will have a 

 complete circulation of its own. A similar movement of fluid 

 has been seen in the Frog-bit (another aquatic plant of this 

 country), and in many others ; and it is nearly certain that it 

 takes place in every vegetable cell that exists, during some period 

 of its growth ; being only visible for a short time in some, which 

 soon arrive at a condition little subject to change ; but continuing 

 during the greater part, or the whole of life, in others. 



357. This movement of fluid in the individual cells, is quite 

 distinct from the general circulation which has been described in 

 the higher plants. It is a part of the process of formation, by 

 which the nutritious fluid that is brought to each part, is con- 

 verted into organised tissue. In the simple Cellular plants, where 



