266 



MICEOSCOPIC FORMS OF VEGETABLE LIFE. 



tliougla in other cases it is merely indicated by a slight notch on 

 one side (as in the marginal cells of Fig. 73, a, e) ; whilst the 

 other is the frequency of projections from their surface, which 

 are sometimes short and inconspicuous (Fig. 75), but are often 

 elongated into spines, presenting a very symmetrical arrangement 

 (Fig. 71). These projections are generally formed by the outer 



Fig. 71. 



-^/^^vj^-n-^i^ 



Various species of Staurasirum : — A, S. vestituni ; B, 5, aculeatum; c, S.paradoxum ; 

 D, c;, S. brachiatum. 



coat alone, which possess an almost horny consistence, so as to 

 retain its form after the discharge of its contents (Figs. 74, e, 75, 

 b), but which does not include any mineral ingredient, either 

 calcareous or siliceous, in its composition. This outer coat is 

 surrounded by a very transparent sheath of gelatinous substance, 

 which is sometimes very distinct (as shown in Fig. 77), whilst in 

 other cases its existence is only indicated by its preventing the 

 contact of the cells. The outer coat encloses an inner mem- 

 brane, which is not always, however, closely adherent to it ; and 

 this immediately surrounds the colored substance which occupies 

 the whole interior of the cell. It is quite certain that the Desmi- 

 diacese, like the Confervoid Plants in general, grow at the ex- 

 pense of the inorganic elements which surround them, instead 

 of depending upon other living beings for their subsistence ; and 

 that they decompose carbonic acid, and give off oxygen, under 

 the influence of sunlight. They have the power of generating 

 from these materials the organic compounds which they require 

 for their own development; and these are such as are formed by 

 other undoubted Protophytes, as is proved by the application of 

 the appropriate tests. Thus the outer coat is colored blue by 

 sulphuric acid and iodine, and is therefore composed of cellulose. 

 The " endochrome" derives its color from the same green sub- 

 stance " chlorophyll," as that which imparts it to Plants gene- 

 rally ; and this is mingled with a protoplasmic fluid, in which 

 " vacuoles" are frequently to be seen. At certain stages in the 



