23.] 



CHAPTER I. THE CELL. 



101 



This view of the significance of the pyrenoids is supported by the fact that 

 starch-grains, i.e. reserves of carbohydrate, are very generally deposited in the 

 chromatophore immediately round the pyrenoid. In most of the Chlorophycese 

 which have pyrenoids, the pyrenoid is (under appropriate conditions) com- 

 pletely enclosed by a sort of shell of starch-grains lying in the adjacent por- 

 tion of the chromatophore (see Fig. 55). In these large chromatophores, the 

 assimilatory and amyloplastic functions are not uniformly distributed : whilst 

 the general mass of the chromatophore carries on the former, the latter is 

 confined to the portion of it which closely invests the pyrenoids. 



Another fact, bearing on the significance of the pyrenoids, is that they vary 

 in bulk from time to time, in relation, apparently, to the condition of nutrition 

 of the cell : hence it is probable that they may yet be found in plants which, at 

 present, seem not to possess them. For instance, the singular fact that they 

 have been discovered in some Diatoms and not in others, may perhaps be ex- 

 plicable on this ground. 



Chromatophores multiply by division into two, effected by 

 median constriction (Figs. 50 B ; 52): pyrenoids, when present, 

 are multiplied in the same way. 



The chloroplastids ultimately undergo degeneration, when, as in 

 the case of falling leaves, for instance, all that remains of them is 

 a few yellow granules. 



In many cases the green colour of parts of plants containing 

 chloroplastids, is masked by the presence 

 of other colouring-matters held in solu- 

 tion in the cell-sap (e.g. the leaves of 

 Amaranthus, Coleus, Copper Beech, Copper 

 Hazel, etc.). 



The Chromoplastids are generally de- 

 rivatives of chloroplastids which have un- 

 dergone a change both in form and colour. 

 They occur most commonly in the cells of 

 yellow floral leaves, such as those of Tro- 

 pseolum (Fig. 56) : in the superficial cells 

 of many fruits of a red or orange colour 

 (e.g. berries of Solanum, fruit of Tomato). 

 The yellow colour of the root of the Carrot 

 is due to the presence of leucoplastids, in 

 each of which there is a large orange- 

 coloured crystal of carotin. The chloro- 

 plastids of many Coniferae (e g. Biota orien- 

 talis) assume a reddish colour at the beginning of winter. 



FIG. 56. From the upper 

 Bide of the calyx of Tropmo- 

 lum majus. The inner wall 

 of an epidermal cell with 

 the chromoplastids. (After 

 Strasburger : x 540.) 



A brief account may be appended of the structures peculiar to primordial 



