32 THE VICTORIAN NATURALIST. 



an up-to-date description of the plants of this family, which 

 is as follows: — Desmids are unicellular fresh-water' plants of 

 microscopic size. They are, in most genera, constricted in 

 the middle to form two more or less conspicuous and equal 

 semi-cells, and multiply by simple vegetative division, and 

 also by means of zygospores, the former asexual and the 

 latter a sexual reproduction of rather low grade. They are, 

 speaking generally, of a uniform grass-green colour, owing to the 

 presence of chlorophyll, contained in bodies known as chloro- 

 plasts, which may be one or more in each semi-cell, clinging to 

 the walls (parietal) or centrally situated (axillary). In the chloro- 

 plasts are imbedded small colourless organs called pyrenoids, 

 which are similar in most respects to those seen in the common 

 " Silk-weed," Spirogyra. 



The great diversity of form, the symmetry, and the beautiful 

 surface markings and other ornamentation of Desmids make 

 them pleasing objects for even casual microscopists, while these 

 characters, taken with others, such as the movement of the plant 

 as a whole and the movement of the contained protoplasm, give 

 to the Desmids an inestimable value in the mind of the botanical 

 student. 



The constriction referred to is distinct in most genera, 

 but less prominent in some, and quite absent in a few. The 

 genus Micrasterias shows it at its maximum, the isthmus 

 joining the inflated portions of the semi-cells being only 

 nominal. The least constriction is seen in the sub-cylindrical 

 genera, such as Penium, Docidium, &c. It is absent from 

 genera such as the crescent-shaped Closterium, the cylindrical 

 Gonatozygon, and others, while in certain species of Staurastrum 

 a peculiar terminal inflation of each semi-cell and abruptly 

 reduced inner part give the appearance of a constriction like a 

 circumcised trench of rectangular section, the whole plant thus 

 resembling approximately a prickly bulbed dumb-bell, as may be 

 seen in one of the microscopic exhibits. In the ]jlants which 

 have no constriction there is a colourless region in the centre of 

 the cell which divides the colouring matter into two parts, and in 

 this locality the nucleus is situated. 



In tracing the descent of Desmids from ancestral filamentous 

 algse of the Conjugatae, West and West draw attention to the 

 genus Penium as showing the first sign of constriction. Some 

 species of that genus have none at all. A further indication of 

 the connection between the free unicellular Desmids and their 

 filamentous ancestors may be seen this evening in a number of 

 exhibits, which partly illustrate West and West's scheme of 

 phylogeny. The genera of which specimens are shown are 

 Spondylosium, showing at least four long cylindrical cells, which 

 have the appearance, at first sight, of a portion of a multicellular 



