MICROSCOPIC FORMS OF VEGETABLE LIFE. 



263 



the fuller knowledge of their life-history, and the more extended ac- 

 quaintance with the parallel histories of other simple forms of Vegeta- 

 tion, which have been gained during the last twenty years, are now gen- 

 erally accepted as decisive of their Vegetable nature. 



261. The DESMIDIACE^; 1 are minute plants of a green color, growing 

 in fresh water; generally 



speaking, the cells are in- Eis. *& 



dependent of each other 



(Figs. 155-158); but some- 

 times those which have 



been produced by binary 

 subdivision from a single 

 primordial cell, remain ad- 

 herent one to another in 

 linear series, so as to form 

 a filament (Fig. 160). 

 The tribe is distinguished 

 by two peculiar features: 

 one of these being the 

 semblance of a division 

 of each cell into two sym- 

 metrical halves by a * su- 

 tural line,' which is some- TT 



Various species of Staurastrum : A, S. vestitum; B, S. aculea- 

 timeS SO decided as tO nave f um; c , 8m paradoxum; D, E, S. brachiatum. 



led to the belief that the cell 



is really double (Fig. 158, A), though in other cases it is merely indicated by 

 a slight notch; whilst the other is the frequency of projections from the 

 surface, which are sometimes short and inconspicuous (Fig. 158), but are 

 often elongated into spines, presenting a very symmetrical arrangement 

 (Fig. 155). These projections are generally formed by the cellulose en- 

 velope alone; which possesses an almost horny consistence, so as to retain 

 its form after the discharge of its contents (Figs. 158, B, D, 162, E), but 

 does not include any mineral ingredient, either calcareous or siliceous, in 

 its composition; in other instances, however, they are formed by a notch- 

 ing of the margin of the cell (Fig. 157) which may affect only the outer 

 casing, or may extend into the cell-cavity. The outer coat is surrounded 

 by a very transparent sheet of gelatinous substance, which is sometimes 

 very distinct (as shown in Fig. 160), whilst in other cases its existence 

 is only indicated by its preventing the contact of the cells. The outer 

 coat incloses a primordial utricle, which is not always closely adherent 

 to it; and this immediately surrounds the endochrome, which occupies 

 nearly the whole interior of the cell, and in certain stages of its growth 

 is found to contain starch-granules. Many of these Plants have a power 

 of slowly changing their place, so that they approach the light side of 

 the vessels in which they are kept, and will even traverse the field of the 

 Microscope under the eye of the observer; by what agency this movement 

 is effected has not yet been certainly made out. 



262. A ' cyclosis' may be readily observed in many Desmidiacece; and 

 is particularly obvious along the convex and concave edges of the cell of 

 any vigorous specimen of Closterium, with a magnifying power of 250 or 



1 Our first accurate knowledge of this group dates from the publication of Mr. 

 Ralfs's admirable Monograph in 1848. Later information in regard to it will be 

 found in the Section contributed by Mr. W. Archer to the 4th Edition of Prit- 

 chard's * Infusoria.' 



