STARCH-GRAINS 411 



number of layers which are laid down either concentrically or excen- 

 trically around a morphological centre or hilum. In intact grains 

 even in those which are very excentric in structure all the layers are 

 complete. Interruption of one or more layers at any point is a sure 

 indication of a preceding partial solution of the grain. The stratified 

 appearance of starch-grains is due, as Nageli first recognised, to differ- 

 ences in the density of successive layers, strata of low water content, 

 and therefore of high density, alternating with layers of low density, 

 which contain a larger proportion of water. 



Different botanists hold very divergent views concerning the 

 ultimate structure of starch-grains. The majority, including A. F. W. 

 Schimper and A. Meyer, regard these bodies as sphaerocrystals 

 (sphaerites) possessed of a radially fibrous structure, or in other words, 

 composed of numerous exceedingly minute radially arranged needle- 

 shaped crystals (trichites). In A. Meyer's opinion, the stratification of 

 the grain is . due to the varying thickness and number, in successive 

 layers, of the more or less richly branched trichites. The alternation 

 of layers of loose and dense texture is further regarded as a conse- 

 quence of periodic fluctuations in the supply of material to the grain 

 during its development. When the chromatophore (amyloplast) in 

 which the grain is being laid down, is well supplied with sugar, it 

 produces a large amount of starch-substance, and the corresponding 

 stratum is therefore relatively dense ; under the opposite conditions a 

 loosely constructed layer is formed. Working with cuttings of Pellionia 

 Daveuana, which were first starved [by darkening] and then once more 

 exposed to bright light, A. Meyer endeavoured to keep count of the 

 new layers laid down, during the second stage of the experiment, upon 

 the starch grains which had been partially dissolved during the period 

 of starvation. He came to the conclusion that a thick, dense layer 

 was formed as the result of each diurnal period of illumination, while 

 the nocturnal intervals of darkness were represented by thin and 

 relatively watery strata. Whether this interpretation is the correct 

 one or not, can scarcely be decided in the present state of our know- 

 ledge. It may, however, be stated that Hugo Fischer has recently 

 contested Meyer's general position. He throws doubt upon the theory 

 of the sphaerocrystalline structure of starch -grains, and suggests that 

 some of the water in the layers of low density is contained in 

 radial crevices, which are obliterated by contraction when the grain 

 is dried. Butschli, finally, believes that starch-grains have an alveolar 

 or foam-like structure, similar to that which he ascribes to proto- 

 plasm. 



It is probable that starch-grains never arise otherwise than in con- 

 nection with chromatophores ; the establishment of this important fact 



