264 FRANCIS DARWIN. 



sible to believe that parasitic organisms would seat themselves al- 

 most without exception on the same part of the gland. Some of the 

 very rare cases in which a filament was seen seated on the side of 

 a trichome proved after all not to be exceptions to the rule; for, 

 by making them contract, they proved to be attached by delicate 

 connecting filaments with the summit of the gland. It is also a 

 very convincing fact that the trichomes of the fig. 1 type never 

 ])roduce a filament, although they are apparently as well fitted as 

 the fig. 2 type for the abode of a parasite. Again, the fact that 

 filaments are found on the leaves of seedlings reared in a hot- 

 house and far from their parent-plants (from which they might 

 otherwise be infected with the supposed parasite) seems to me 

 strongly against the parasite hypothesis. 



Putting aside any view of this nature, we seem to be reduced 

 to two theories — (1) That the filaments are protrusions of the 

 resinous protoplasm of the glands. (2) That they consist of 

 a resinous secretion of gelatinous consistence; and that the 

 movements which occur are not due to vital activity inherent in 

 the substance of the filaments, but are due to purely mechanical 

 causes. The movements might be supposed to be similar in 

 kind to those observed by Professor Eay Lankester^ in the 

 coloured blood-corpuscles. He found that the merest trace of 

 the vapour of ammonia caused a wrinkled wave to travel over 

 the surface of the corpuscle, simulating contraction. A stronger 

 dose causes the protrusion from the corpuscle of processes which 

 collapse when acetic acid is substituted for ammonia. These 

 movements are believed by Professor Lankester to be purely 

 mechanical in nature. As another instance of purely physical 

 effects closely similar to " vital " movements may be cited 

 Sachs' recent research, ' Ueber die Emulsion Eiguren, &c."^ 

 Previous investigators had believed the movements of swarm- 

 spores in relation to light were truly vital in nature. Sachs has, 

 however, proved, by obtaining similar phenomena with enmlsions 

 of oil, that they are the direct physical result of slight difi'er- 

 ences in the temperature of surrounding objects. 



I shall now briefly consider the behaviour of the filaments in 

 relation to the above-mentioned theories, which may be called 

 (1) the vital; (2) the mechanical theory. 



In favour of the mechanical theorij we have the fact that the 

 filaments undoubtedly contain a large percentage of resinous 

 matter, which might conceivably give rise to mechanical pseudo- 

 contractions. Again, it might seem that there is a greater 

 a priori probability of such pseudo-contractions occurring than 

 of the protrusion of a highly resinous protoplasm through the 



' 'Quarterly Journal of Microscopical Science,' 1S71, p- 376. 

 2 ' J^lora,' Nrs. 16, 17, IS, 1876. 



