S8 ANATOMY AND PHYSIOLOGY OF 



As the cell increases in size, its membrane grows in much greater 

 proportion than the nncleus, which certainly frequently enlarges 

 for a certain time, but becomes smaller in propoition to the cell. 

 During the growth of the cell irregularly scattered cavities are 

 formed in the protoplasm ; these are originally isolated, and very 

 frequently present a most deceptive resemblance to cavities of 

 delicate- walled cells, subsequently, however, they become blended 

 together in many directions ; the protoplasm is then accumulated 

 at one side, in the vicinity of the nucleus ; on the other side it 

 coats the inside of the primordial utricle, and these two collections 

 are connected together by thread-like processes which are some- 

 times simple and sometimes branchecl, so that the nucleus appears 

 suspended, as in a spider's web, in the centre of the cell.* An 

 internal movement in the protoplasm now begins to be visible. 

 Originally no definite arrangement can be perceived in it ; but 

 the more the protoplasm changes from the uniform mass which 

 it originally formed, into the condition of threads, the more dis- 

 tinctly it may be seen that each of these threads represents a 

 thinner or thicker stream, which in one thread flows from the 

 nucleus to the periphery, turns round there, and flows back a2:ain 

 m another thread. The thickness, the position, and the number 

 of these threads are subject to constant change, which shews, 

 beyond a doubt, that the currents move freely through the watery 

 cell-sap, and are not enclosed in membranous canals. In most 

 cases the nucleus does not appear to take any part in this move- 

 ment ; but the motion may easily be overlooked on account of its 

 slowness, since I found in Tradescantia virginica, in which T saw 

 the nucleus move slowly up and down, that this only passed over 

 the distance of 1 -45,000th of a line in a second, which is naturally 

 much too little to allow of the movement being seen directly, 

 even by the application of the strongest magnifying powers. The 

 nucleus retains its central position in many cases even when the 

 cell is fully developed, e. g., in Zygnema, but it mostly becomes 

 gradually withdrawn towards one side of the wall of the cell, where 

 it becomes attached by its viscid investment to the primordial 

 utricle, bxxt always forms the centre of the currents of sap. The 

 Circulation of the protoplasm is very slow ; I determined it in the 

 hairs of the filaments of Tradescantia at an average of l-500th of 

 a line per second, in the stinging hairs of UHica bacciferal -7 50tl\ 

 in the hairs of Gucurhita Pepo at l-1857th, &c. (''Bot Zeitimgf' 

 1846, 92.) 



In most cells this phenomenon is transitory, for not only is the 

 nucleus itself dissolved in time in the majority of cases, but the 

 protoplasm also becomes more and more diminished in quantity, 

 or at least frequently appears motionless, as appears in all proba- 



* PI 1, fig. 7. The end tjell of a hair of the filament of Tradesccmtla 

 jSeUoivii, 



