i8 PRACTICAL BOTANY 



This Alga is commonly to be found in summer, growing in 

 stagnant, or slowly flowing, fresh water : it is to be recognised 

 by its bright green colour, and the unbranched filaments 

 irregularly coiled together, and of sufficient size to be dis- 

 tinguished by the naked eye : the whole flocculent mass feels 

 slimy when lifted from the water. 



Mount a small quantity of the Spirogyra in water : examine 

 it first under a low power, and observe on one of the largest 

 specimens 



1, That the cylindrical filament is limited by a definite 

 cell-wall. 



2, That transverse partitions septa which are continuous 

 with the limiting cell-wall, divide the filament into a linear 

 series of cells. 



3, That each cell contains a protoplasmic body, the most 

 marked part of which will be one or more spiral chromato 

 phores, coloured bright green by chlorophyll. 



In order to study the structure of the protoplasmic body in 

 detail, put on. a high power, and, focusing carefully, make 

 the following observations of the filament, which, it is to be 

 remembered, is in the living condition : 



1. Having recognized the smooth colourless cell-wall, note 

 that it is closely invested internally by 



2. A continuous film of colourless protoplasm (the " primor- 

 dial utricle "). 



3. That this protoplasmic film surrounds a large central 

 cavity the vacuole filled with perfectly transparent cell-sap. 



4. That in the peripheral film are embedded one or more 

 green spiral bodies (chromatophores) of flattened form, and 

 very irregular margin, each including numerous lenticular, 

 highly refractive bodies, the pyrenoids. 



5. Focusing carefully downwards into the central cavity, a 

 highly refractive, colourless, lens-shaped body is to be seen 

 suspended in a central position by numerous finely granular 

 protoplasmic threads : this body is the nucleus. 



a. These several points should be made out in the living 

 cell, without treatment with any reagent ; but their observation 

 may be made easier in various ways. Irrigate (p. u) with 



