108 INTRODUCTION TO CYTOLOGY 



granules of paramylum in Euglena and those of "Floridean starch" in 

 the red alga? first appear in the cytoplasm; but, although they arc the 

 first substances which are visible, it is highly probable that they arise 

 through the transformation of a non-visible product (sugar?) of the 

 photosynthetic activity of the plastids, and are not immediately built 

 up from water and carbon dioxide. A similar interpretation may be 

 placed upon corresponding appearances reported in the case of higher 

 plants. Owing to the great difficulty of determining the true cell struc- 

 ture of the Cyanophyceae (see p. 202) it is possible to speak of plastid 

 activity in such forms only with great reserve. If, as Olive (1904) and 

 Gardner (1906) hold, these cells are without plastids, the product of 

 photosynthetic activity, commonly glycogen, must be elaborated in the 

 cytoplasm without their aid. If, on the other hand, the peripheral 

 portion of the protoplast represents a large chromatophore (Fischer 1898), 

 or cytoplasm containing a large number of minute chromatophores 

 (Hegler 1901, Kohl 1903, Wager 1903), the photosynthetic process, 

 although it may result in the production of a different substance, is 

 dependent upon the powers of definite protoplasmic organs much the 

 same as in higher plants. Among bacteria and other low forms in which 

 it seems more certain that plastids and the ordinary pigments are absent, 

 widely different types of metabolism are met with. For further discus- 

 sion of this subject, which lies outside the scope of the present book, 

 more special physiological works should be consulted. 



The Pyrenoid. The term pyrenoid was applied by Schmitz (1882) 

 to the refractive kernel-like bodies imbedded in the chromatophores of the 

 algae. Pyrenoids are characteristic of the Chlorophycese especially, 

 being present almost universally in the members of this group. They 

 are known in a few representatives of the IJiodophyceae (Nemalion 

 and the Bangiacese), but apparently do not occur in the cells of the 

 Cyanophyceae, Phaeophyceae, and Characeae. Very rarely they are 

 present in forms above the algae : a conspicuous example is the liverwort 

 Anthoceros. The chromatophore may contain but one pyrenoid, as in 

 Zygnema, or a larger number, as in Spirogyra, Draparnaldia, and many 

 other forms (Fig. 37). 



As held by de Bary (1858), Schmitz (1884), and Schimper (1885), 

 the pyrenoid appears to be composed of a protein substance with a thick 

 gelatinous consistency. When a single pyrenoid is present in the chroma- 

 tophore it may multiply by fission along with the latter when the cell 

 divides, while in those forms possessing several pyrenoids this multiplica- 

 tion may be much more extensive. Also, as pointed out by Schmitz 

 and Schimper, and more recently by Smith (1914), the pyrenoid may 

 disappear and arise de novo from the cytoplasm or from the plastid 

 protoplasm. 



With regard to its function, the early workers referred to above ob- 



