36 THE CELL 



among Algae and Fungi and in all the higher groups of plants, but also 

 throughout the animal kingdom ; it is therefore generally believed that 

 the nuclei are homologous in all these various phyla. It is only in 

 the- very lowly organised Schizophyta (Cyanophyceae and Bacteria) 

 that nuclei of the normal type have not as yet been identified. 

 Whether the so-called chromatin-granules which occur in Bacteria 

 partake of the nature of nuclei or not, is quite uncertain. The " central 

 body " of the Cyanophyceae, which has been studied in detail by 

 Zacharias, Biitschli and Palla among others, also differs in so many 

 points of structure and behaviour from a typical nucleus, that the two 

 organs cannot be homologized. 15 It is nevertheless conceivable 

 that this central body may perform at any rate some of the functions 

 of a nucleus, and that it may even stand in a certain remote 

 phylogenetic relation to the typical nucleus of higher organisms. 



4. Chromatophores. 



The chromatophores are characteristic protoplasmic organs, which 

 are peculiar to vegetable cells. They occur in all the great groups of 

 plants, with the exception of the Fungi, Bacteria, and Myxomycetes. 

 Chromatophores are not, however, necessarily present in all the cells 

 and tissues of the plant- body, and they are abundantly developed only 

 in tissues which are concerned with certain special physiological or 

 ecological functions. Chromatophores are distinguished from other 

 protoplasmic organs by the fact that they contain characteristic pig- 

 ments, or, at any rate, possess the capacity of assuming a pigmented 

 condition in certain circumstances. Since A. F. W. Schimper's classical 

 researches on the subject, it has been customary to recognise three 

 varieties of chromatophores, namely, chloroplasts, chromoplasts, and 

 leucoplasts. 16 



Chloroplasts owe their characteristic colour to the presence of a 

 green pigment, chlorophyll, which is accompanied by small quantities 

 of yellow colouring matters, or xanthophylls. In those Algae which 

 are not green in colour, chlorophyll is nevertheless present, but is 

 masked by accessory pigments. Among Algae the shape of the chloro- 

 plasts varies greatly ; the Higher Plants, on the other hand, almost 

 invariably possess small discoid " chlorophyll-corpuscles." Chloroplasts 

 are structures of the highest physiological importance. They are the 

 actual photosynthetic organs of the cell, and hence attain their most 

 characteristic development in specialised photosynthetic tissues, although 

 they occur in smaller numbers in many other tissues as well. Since, 

 however, they are not essential components of every unspecialised 

 plant cell, they will not be discussed in detail until the photosynthetic 

 system is under consideration (cf. Chap. VI.). 



