498 



PHYSIOLOGY. 



by the contraction of the protoplasmic threads attached to it, and, 

 moreover, itself undergoes, in some cases, changes of form ana- 

 logous to those manifested by Amcelce. (See under Myocomycetes, 

 p. 471.) 



The gradual vacuolation of the protoplasm and the transfer of the nu- 

 cleus to the side of the cell may be well seen in the hairs of the stamens 

 of Tradescantia (fig. 547). In Spirogyra and Zygnema the nucleus re- 

 mains always suspended in the middle of the cell by the protoplasmic 

 cords. The ultimately parietal nucleus of the hairs of Tradescantia ex- 

 hibits radiating cords, the protoplasm here being in process o^absorption. 

 In Vallisneria and in CEdogonium and other Confervoids, the nucleus 

 becomes imbedded in the continuous parietal layer of protoplasm which 

 lies upon the primordial utricle. The nucleus has the property of break- 

 ing up and, as it were, disappearing for a time, to reappear in the form of 

 two or more new nuclei of larger size than the original nucleus. This 

 process occurs in the formation of the pollen in the embryo-sac of Phane- 

 rogamous plants, &c. The nucleus is sometimes, according to Cohn, in- 

 vested with a layer of starch, and is coloured by carmine solution, which 

 leaves the amylaceous envelope unstained. 



Chlorophyll. In all parts of plants which have Fig. 549. 



a green colour we find the cell containing in its 

 cavity structures quite distinct from the cell-wall 

 and from the primordial utricle, in which the green 

 colouring-matter resides. The ordinary form of 

 these is that of globular or spheroidal corpuscles, 

 which appear in greater number and of darker 

 green colour in proportion to the intensity of solar 

 light to which the tissue may be exposed. In a 

 few cases the green colouring-matter is found in 

 the form of annular or spiral bands (Draparnaldia, 

 Spirogyra, fig. 549), or of reticulated cords (Clado- 

 pJiora), of mucilaginous consistence, adhering to 

 the^inside of the primordial utricle. In some Con- 

 fervse the green colouring-matter appears diffused 

 through a portion of the protoplasm in the form of 

 very minute granules. In many unicellular Algae, 

 in the Algoid gonidia of Lichens, &c., the green 

 colouring-matter is uniformly distributed through- 

 out the cell, and is not separable from the rest of 

 the protoplasm. 



The Chlorophyll-corpuscles are of soft consistence ; 

 and their colour is extracted bv ether, alcohol, and , 



. , mi . . - , ' . ' Cells of a filament 



various acids. I hey consist 01 protoplasmic colour- of Spiropyra, with 

 less substance mixed with colouring-matter. The 

 former may exist by itself unmixed ; but the colour- 

 ing-matter is never found separate in nature. They appear 



spiral green bands. 

 Magn. 200 diain. 



