164 The Sense-Organs of Plants. [Sess. 



X.—TEE SENSE-ORGANS OF PLANTS 

 By Mr R A. ROBERTSON, M.A., B.Sc, F.R.S.E., F.L.S. 



{Communicated Oct. 27, 1909.) 



In primitive organisms such as the Flagellata, which may be 

 taken as the phylogenetic starting-point of both plant and 

 animal descent, active locomotor movements are characteristic 

 of the dominant phase in the life-history. When the path 

 of evolution is traced onwards, a noteworthy feature appears, 

 namely, that while animals retain throughout this primitive 

 motility, plants on the other hand gradually lose it, becoming 

 stationary in the dominant phase, while the motile phase is 

 found as a relic of evolution only in the reproductive stages, 

 and even here in the highest plants it is lost entirely. Those 

 locomotor movements are automatic, but they may be altered 

 in direction and velocity by environmental changes. The 

 chemo tactic and photo tactic phenomena exhibited by uni- 

 cellular organisms — by gametes or spermatozoids — prove that 

 environmental changes are perceived by the organism, and 

 apparently by no part of the unit in a higher degree than 

 any other part. Within limits this is still true even of higher 

 organisms, as for example when the perception of touch or 

 contact is found to be general rather than local. Increasing 

 specialisation, however, brings with it physiological division 

 of labour, and the perception of particular changes comes to 

 be localised most acutely in special areas of the superficies 

 known as sense-organs. 



Although plants have gradually lost the power of active 

 locomotor movement, they have, in common with animals, 

 slow movements of growth. In the earliest unicellular phase 

 of plant or animal — the egg — as well as in the multicellular 

 embryonic phases, the whole organism is implicated in this 

 movement ; but as maturity is attained, the growth movements 

 in plants are confined to certain areas of the organs, — the 

 terminal regions of shoot and root, for example. The growth 

 movements are initiated naturally in the egg by the stimulus 

 of conjugation with the spermatozoid, and in many plant eggs 



