6i6 



LECTURE XXXV. 



closely applied to the cell-wall, remains as it appears relatively at rest. Having 



already (loc. cU) characterised, in, the 

 main the form of movement, and 

 since a detailed description would 

 not give the reader any notion of the 

 motile forces, I. may here confine my- 

 self to mentioning briefly the few facts 

 which we know concerning the irrita-, 

 bility of the circulating protoplasm, and 

 fundamentally this will apply also to 

 the rarer form of movement, the Ro- 

 tation of the protoplasm, which is alsq 

 mentioned in the same place. Whether 

 and how far gravitatioji -may in any, 

 way influence the movements is not 

 known. That rotation i and circula- 

 tion take place even in profound and 

 continued darkness, and likewise in 

 coloured .light, and that by iUumina^- 

 tion under the microscope at least no 

 striking change is undergone, is well 

 known', although this does not ex- 

 clude the possibility that more exact 

 studies in this, direction may demon- 

 strate an irritability to variations in 

 the light. Considering the great seni 

 sitiveness of swarm-spores and plasr 

 modia for light, it is hardly credible 

 that the ■ protoplasm within the cells 

 should be indifferent towards it; more- 

 over ajl heliotropic organs, which are 

 thus sensitive to light, of course con- 

 tain protoplasm in their cells, and 

 we ha,ve every reason to believe that 

 the light-stimulus in heUotropic organs 

 aifects principally their protoplasm, 

 and that the corresponding alterations 

 in the cell-walls are initiated by this. 

 It is thus obvious (and the reason- 

 ing applies to geotropic . organs also) 

 that aU protoplasm enclosed in cells 

 is irritable to gravitation and light, 

 only of course in a manner not 



Fig. 361.— Optical longitudinal section of the middle cell of 

 hair of the Gourd (from the calyx of a youn^ flower-bud). Cell- 

 wall simply in outline — the fine granules in the protoplasm 

 drawn too coarse. The central vacuolated clump encloses the 

 nucleus of the cell. The streaming filaments, everywhere in 

 active movement, carry chlorophyll -corpuscles (containing 

 stanch) in their substance : at one place (to the left) a crystal 

 is also carried along. 



' In the Bot. Zeitg. 1863 (Supplement, p. 3) I stated that the protoplasm circulates even in the 

 cells of etiolated organs, e. g. hairs of Cucurbita : in feet, I prefer to employ wholly or partially 

 etiolated plants for the demonstration of protoplasmic movements, on account of several advantages. 



