ARRANGEMENT OF CHLOROPLASTS. 269 



Iris the chromatophores present a peculiar granular appearance in 

 cells of all ages, owing to the presence of innumerable minute drops of 

 oil. In general, such oily inclusions may be regarded as waste- 

 products produced in consequence of a senile " fatty degeneration " of 

 the chloroplast ; but it is possible that in certain cases (such as that of 

 Vaucheria) they represent the normal product of photosynthesis. 



Protein-crystals (crystalloids) have been observed in the chloroplasts 

 of certain plants (Phaius grandifolius, Zingiber officinale, Pellionia 

 Daveauana, etc.), where they seem to play the part of reserve- 

 materials. Mention must further be made of the pyrenoids, which 

 form a characteristic feature of the chloroplasts among Algae, and also 

 in the genus Anthoceros. According to Schimper, a pyrenoid often 

 consists of a single protein-crystal, which may or may not be enveloped 

 in a layer of substance resembling protein. In certain cases (Zygnema, 

 Cosmarium) the pyrenoids are not crystalline. Pyrenoids are fre- 

 quently surrounded by numerous small starch-grains arranged in the 

 form of concentric shells. 



2. Arrangement of chloroplasts in the cell. 141 



As regards their arrangement in the individual cell, chloroplasts 

 usually exhibit definite relations to the other components of the proto- 

 plast, and to the various regions of the cell-wall ; their disposition is, in 

 fact, in the first instance dependent upon certain internal factors, which 

 are more or less directly connected with the photosynthetic function. 

 As a rule, however, the arrangement of the chloroplasts is by no means 

 absolutely determined by these internal relations, but is, on the contrary, 

 constantly subject to alteration under the influence of various external 

 stimuli, among which illumination plays the leading part. The result- 

 ing movements of the chloroplasts generally react in a favourable 

 manner upon the activity of the chlorophyll apparatus. 



Beginning with the internal factors that affect the arrangement of 

 chloroplasts, we must note that these bodies almost invariably take up 

 a peripheral position in the cell ; in this case they always form a single 

 layer, which is closely appressed to the cell-wall, or rather to the 

 ectoplast. This peripheral location not only enables the chloroplasts 

 to utilise the light that reaches them as fully as possible, but also 

 facilitates the gaseous interchanges associated with the photosynthetic 

 function. The latter consideration further explains why, in the photo- 

 synthetic tissues of Higher Plants, the chloroplasts if they are not 

 too closely packed adhere exclusively, or in great part, to those walls 

 which abut upon air-spaces ; by this means they evidently obtain the 

 most favourable conditions for the absorption of carbon-dioxide. 14 ' 2 

 The last-mentioned point can be readily verified in the case of the 



