SPIROGYRA. 253 



take a dark brown colour, and the irregularly serrate or jagged edges 

 of the chlorophyll bands become very distinct. The central nucleus 

 in this species is spindle-shaped, but by pressure upon the cell it is 

 forced out of position, and shows a side view, and then presents 

 the form of a disk ; it has, therefore, in reality the form of a bi- 

 convex lens. In its centre lies a large distinct nucleolus ; sel- 

 dom two or three such bodies are distributed symmetrically in 

 the interior of the nucleus. In other nearly allied species the 

 nucleus is thicker, and appears, in its natural position in the cell, 

 to be rectangular, with rounded corners. The nucleus is sur- 

 rounded by a very thin layer of protoplasm, from which delicate 

 protoplasmic threads run out towards the peripheral protoplasm 

 of the cell. By these threads the nucleus is suspended in the 

 cell- sap which fills the cavity of the cell. The threads all arise 

 from the thin rim of the nucleus, usually fork repeatedly in their 

 course and join on to the inner side of the chlorophyll bands, and 

 in all cases at the projecting parts which cover the pyrenoids. We 

 can readily convince ourselves of this in most cases by slowly 

 changing the focus. In individual cells the nucleus may have 

 a position not central. This may indicate a late stage of cell- 

 division, in which the new r cell- wall shows clearly, with its thick 

 cytoplasmic layer on each side, and the two daughter nuclei lying 

 very near to it on either side. 



Plasmolytic Studies in Spirogyra. From the striking char- 

 acter and distribution of its cell-contents, Spirogyra offers a very 

 favourable material for the study of plasmolysis, in which not 

 merely the ordinary phenomena, but the remarkable capacity for 

 resistance of the vacuolar protoplasmic membrane, is displayed. 

 If a 10 per cent, solution of saltpetre, which is coloured with a little 

 eosin, is permitted to act slowly upon this alga, in most cells a 

 normal plasmolysis is seen to be induced. The protoplasm with- 

 draws at first from the angles, then from the end surfaces, and 

 ultimately from the side- walls, and contracts into a more or less 

 spherical body. The eosin colour passes through the cell- wall up 

 to the contracted protoplasm, but does not at first penetrate it. 

 This is due to the resistance of the outer protoplasmic membrane, 

 the ectoplasm. After from one to two hours, but perhaps sooner, 

 the outer layers of the protoplasm begin to die, and to then 

 accumulate the colouring matter. The wall of the vacuole, en- 

 closing the now concentrated cell- sap, shows, however, still no 

 change, and is impassable by the eosin ; and the vacuoles are 



