224 PRACTICAL BOTANY 



1. That the whole filament is covered externally by a trans- 

 parent gelatinous sheath, with a somewhat irregular outer 

 surface, and showing a radial striation. It is to this layer that 

 the Alga owes its slimy character. It is to be noted, however, 

 that this sheath is almost entirely absent in some species. 



2. A firm cell-wail, which is more highly refractive : it 

 immediately surrounds the protoplasmic body, and is continuous 

 with the transverse septa. 



3. The protoplasmic body, which consists of 



a. A colourless membrane (primordial utricle) which lines 

 the cell-wall internally, and surrounds the large central vacuole. 



b. The green spiral chromatophores (one or more) embedded 

 in the protoplasm : note their irregular outline, and the 

 numerous highly refractive lenticular bodies (pyrenoids) 

 which are contained in them. 



c. A bi-convex lens-shaped nucleus, suspended in the centre 

 of the vacuole by fine colourless strands of protoplasm, which 

 run to the primordial utricle, attaching themselves to points 

 opposite the pyrenoids. 



Stain with an iodine solution, and observe that the colourless 

 protoplasm stains pale yellow, the nucleus a deeper yellow, and 

 it will thus be more clearly seen, as well as one or sometimes 

 two nucleoli which are deeply stained: the pyrenoids stain 

 a dusky purple. 



II. The process of cell-division may be very well observed in 

 the filaments of Spirogyra; the chief difficulty is however that 

 the process normally takes place at night, beginning about 10 

 to 12 P.M. If the Spirogyra be placed in a flat plate upon 

 a block of ice during the night, and on the following morning 

 the plants be exposed to a higher temperature, the cell-division 

 which had been previously retarded will begin, and the 

 successive stages may be followed. 



The following points in the process should be specially 

 noted 



1. The disappearance of the nucleolus. 



2. The formation of the striated nuclear spindle and of the 

 equatorial nuclear disk. 



3. Division of the chromatin which constitutes the disk, and 



