88 PRACTICAL BOTANY. 



different filaments there is considerable variety in size, 

 form of the cells, thickness of the walls, arid in the 

 number and arrangement of the spirals : according to 

 these characters (together with those of the zygospore) 

 a large number of species of Spirogyra are distin- 

 guished. It will be found convenient to select for 

 observation specimens of the largest size, and with the 

 coils of the green spirals furthest apart. Examine 

 such filaments in detail under a high power, in the 

 living state, mounted in water, and observe 



1. That the whole filament is covered externally by 

 a transparent gelatinous sheath, with a somewhat 

 irregular outer surface, and showing a radial striation : 

 this is stated by Klebs to be an excretion from the 

 protoplasm, not a result of metamorphosis of the outer 

 layer of the cell-wall. It is to this layer that the Alga 

 owes its slimy character. 



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

 it immediately surrounds the protoplasmic body, and is 

 continuous with the transverse septa. 



In respect of the nature of the septa there is some variety in 

 different species, and the whole genus may accordingly be divided 

 into two sections : (i.) those in which the septa appear as simple 

 lamellae, and the ends of the cells are then truncate ; (ii.) those in 

 which the septum is split in the central part of its area into two 

 layers, which separate from one another and appear as two invo- 

 lutions, encroaching on the cavity of the cells. A sort of vegeta- 

 tive multiplication of these plants takes place by breaking up of 

 the filaments, accompanied by splitting of the septa : in the species 

 of section (ii.) the involute walls are then pressed outwards by the 

 turgescence of the cells, and become convex : this may often be 

 observed by moving the cover-slip while gently pressing on the 

 object. 



