CHLOROPLASTS. 265 



chloroplast. Schimper has, however, pointed out that the first indi- 

 cations of a fragmentation of the primitive single chloroplasts may 

 be observed in the sporogonium of this genus ; here most of the 

 cells have two chloroplasts, while several may occur in each of the 

 epidermal elements. The only other exception hitherto recorded 

 among Higher Plants is furnished by the genus Selaginella. The 

 author has himself shown that each of the funnel-shaped photosynthetic 

 cells of Selaginella Martensii and S. grandis possesses but a single large 

 bowl-shaped chloroplast, which completely covers the cell-wall in the 

 lower half of the cell (Fig. 106 a). In S. Kraussiana there may be 

 either a single chloroplast or a pair of these structures in each green 

 cell. In S. cacsia, finally, every green cell contains two chloroplasts, 

 the arrangement of which leaves no doubt that they correspond to 

 the two halves of a single bowl-shaped chromatophore (Fig. 106 b) 



A B 



r rnr 



mm Wm 



i 



Fig. 100. 



A. Photosynthetic cell of Si lagineUa Marti nsii, with a solitary bowl-shaped chloro- 

 plast. B. Two photosynthetic cells of S. caesia, each containing a pair of chloro- 

 plasts. 



It should further be noted that, towards the base of the leaf, the 

 chloroplasts are more or less deeply constricted, or even divided 

 into several distinct portions. Here, as in Anthoceros, there is an 

 unmistakable tendency for an originally solitary chloroplast to 

 undergo fragmentation. 



In reviewing the various forms of chloroplasts that have been 

 described one receives the impression that Nature has, as it were, 

 experimented among the lower green plants in order first to test the 

 efficiency of the various modifications, and then to select the best 

 pattern for further elaboration among the Higher Plants. It is, as a 

 matter of fact, easy to show that the form of chlorophyll apparatus 

 which is characterised by the presence of numerous small chloroplasts 

 in each photosynthetic element has many advantages over other types. 

 The immediate duty of every active chloroplast is the absorption of the 

 carbon-dioxide that diffuses into the cell. Other things being equal, the 

 rapidity and efficiency with which this task can be performed depends 

 upon the surface available for purposes of absorption, and obviously the 

 maximum exposure of surface will be obtained when the functions of the 



