72 



PART H. ANATOMY AND HISTOLOGY. 



23 



Chromatophores multiply by division into two, effected by 

 median constriction (Figs. 38 B ; 41) : 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 solution in the .cell-sap (e.g. the leaves of Amaranthus, 

 Coleus, Copper Beech, Copper Hazel, etc.). 



The Chromoplastids are generally derivatives of chroi 

 which have undergone a change both in form and colour. They 

 occur most commonly in the cells of yellow floral leaves, such as 

 those of Tropseolum (Fig. 42) : in the super- 

 ficial 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 leuco- 

 plastids, in each of which there is a large 

 orange-coloured crystal of carotin. The 

 chloroplastids of many Coniferse (e.g. Biota 

 orientalis) assume a reddish colour at the 

 beginning of winter. 



Many of the primordial reproductive cells are 

 motile (zoospores, planogametes, spermatozoids), 

 and move by means of cilia. A cilium is a 

 delicate filament of protoplasm which is con- 

 tractile, and by its oscillations serves to propel 

 through the water the body to which it be- 

 longs. The number of cilia borne by these cells 

 varies considerably: there may be a single 

 cilium (e.g. zoospores of Botrydium, and occa- 

 sionally those of Hydrodictyon) : or a pair (generally in planogametes ; 

 frequently in zoospores ; less commonly in spermatozoids, as those of most 

 heterogamous Algae, of the Bryophyta, and of Lycopodium and Selagin- 

 ella): or four (e. a . zoospores of certain green Algae, Ulothrix, Cladophora, 

 Ulva); or many (e.g. all motile cells of (Edogonium ; zoospores of 

 Vaucheria; spermatozoids of Filicinse and Equisetinae). Cilia also occur 

 in free-swimming Algae, such as Volvox, etc. 



23. The Cell- Wai I is a non-protoplasmic membrane_con- 

 sisting, at least at its first formation, of an organic substance 

 termed cellulose, of water, and of a small proportion of Inorganic 



FIG. 42. From the upper 

 side of the calyx of Trojxeo- 

 lum majut. The inner wall 

 of an epidermal cell with 

 the chromoplastids. (After 

 Strasburger : x 540.) 



