6i6 



LECTURE XXXV. 



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



already {/oc. ctt.) 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 also 

 mentioned in the same place. Whether 

 and how far gravitation may in any 

 way influence the movements is not 

 known. That rotation and circula- 

 tion take place even in profound and 

 continued darkness, and likewise in 

 coloured light, and that by illumina- 

 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 sen- 

 sitiveness of swarm-spores and plas- 

 modia for light, it is hardly credible 

 that the protoplasm within the cells 

 should be indißerent towards it; more- 

 over all heliotropic organs, which are 

 thus sensitive to light, of course con- 

 tain protoplasm in their cells, and 

 we have every reason to believe that 

 the light-stimulus in heliotropic organs 

 affects 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 all protoplasm enclosed in cells 

 is irritable to gravitation and light, 

 only of course in a manner not 



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

 hair of the Gourd (from the calyx of a young 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 

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

 is also carried along. 



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

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

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



