100 



PART II. THE INTIMATE STRUCTURE OF PLANTS. 



[23. 



oilj solution, and to be confined to the fibrillar portions of the 

 plastid, in the form of droplets (grana). 



The most common form of chloroplastid the only one occurring 

 in the higher plants is the chlorophyll- corpuscle (Fig. 52), which 

 is flattened and discoid. Usually, many corpuscles are present in 

 a cell, but occasionally (e.g. Anthoceros) there is only one. In 

 the Algae the chroinatophores, though sometimes small and 

 discoid (e.g. Vaucheria, Fucus, etc.), are more commonly large, 

 occurring singly, and of very various form. 



Fio. 51. Spirogyra majuscula (after Stras- 

 burger : x 240). A cell of a filament, showing 

 the nucleus suspended in the centre; also 

 the spirally-wound chromatophore with py- 

 renoids. 



Fio. 55. Part of a cell of Spirogyra 

 showing the chromatophore c?i, with 

 the pyrenoids p, surrounded by a dense 

 layer of protoplasm in which are nu- 

 merous starch-grains. (After Schmitz : 

 x 800.) 



The chromatophores of the Algse present a great variety of form. Generally 

 speaking, those of the higher forms are small corpuscles of a more or less 

 discoid form ; while in the lower forms the chromatophores are few in number, 

 often single, in each cell, and are relatively large, assuming commonly the 

 shape of a flattened plate, sometimes elongated and straight or spirally coiled 

 (Figs. 53, 54). In the latter case the large flattened chromatophores present 

 one or more spherical thickenings, each of which is termed a pyrenoid (Figs. 

 53, 55), and consists of a homogenous colourless mass of proteid substance. 

 From their limitation to the lowest forms of Algae (some Diatoms, among 

 Phajophyceae ; Bangiaceae, Nemalieae, among Khodophyceae ; Conjugates, Con- 

 iervoideas, among Chlorophyceae), it would appear that there is some connexion 

 between the presence of pyrenoids and an incomplete differentiation of repi'o- 

 ductive and vegetative cells in the plant in which the pyrenoids occur (see 

 Alga, Part III.). They may, in fact, be regarded as masses of reserve proteid. 

 The only plant, outside the Algae, in which a pyrenoid has been found in the 

 chrcmatophore, is Anthoceros. 



