26 LECTURE II. 



(chlorophyll or etiolin) or may be colourless ; the former are 

 termed chlorophyll- (or etiolin-} corpuscles, the latter starch- 

 forming-corpuscles or amyloplasts. Their structure and general 

 chemical composition are probably the same as those of the 

 protoplasm. 



A few words may be added here with reference to the microsomata. 

 These are minute particles of an irregularly rounded or somewhat 

 elongated form ; they stain readily, and are evidently of a protoplasmic 

 nature. As already mentioned, they occur especially in the endoplasm, 

 but they are to be found occasionally in the ectoplasm. Schmitz has 

 observed that in protoplasm, which presents a distinct fibrillar reticulum, 

 the microsomata appear to be attached to the fibrillae : he compares them 

 to the chromatin-granules of the nucleus (see infra). They play an 

 important part in connexion with the formation of the cell-wall. When a 

 mass of protoplasm is about to surround itself with a cell-wall, the ecto- 

 plasm becomes filled with microsomata, and similarly in cell-division the 

 cell-plate, from which the wall is formed, is made up of microsomata. 

 The microsomata coalesce laterally and become altered into cellulose. 

 This is clearly shewn in the case of cell-walls which present oblique 

 striation. Strasburger observed in the finely striated cells of Pinus syl- 

 vestris that the microsomata are arranged in spiral rows, corresponding 

 to the planes of striation seen in the cell-wall, and it appears that, in the 

 formation of a layer of the cell-wall, the microsomata of each row 

 coalesce to form a spiral band of cellulose ; in more coarsely striated 

 cell- walls, several adjacent rows of microsomata coalesce laterally to form 

 a single spiral band. 



The Nucleus. 



The nucleus (Fig. 3) is a body of a somewhat oval form 

 which can be readily distinguished, on account of its being 

 more highly refractive, from the protoplasm by which it is 

 surrounded. Its outline is definitely marked owing to its 

 structure being more firm towards the periphery. In the 

 interior of the nucleus there is a distinct rounded body the 

 nucleolus; it frequently happens that two or more nucleoli 

 are present. Besides this, the ground-substance of the 

 nucleus usually contains a number of granules, but in some 

 cases it can be made out that it is traversed in all directions 

 by trabecular fibres, which form a reticular frame-work within 

 it, the meshes of which are larger towards the centre and 

 smaller towards the periphery. 



