MOTILE MECHANISMS IN HIGHER 



PLANTS 



By J. BRETLAND FARMER, D.Sc, F.R.S. 



Many persons who are in the habit of observing natural pheno- 

 mena must have been occasionally struck by the rapid change in 

 the appearance of a field of daisies when the sunlight becomes 

 obscured by heavy clouds. The previously fully-expanded 

 flowers " close up " rapidly. This is owing to the incurving of 

 the green bracts which enclose the cluster of small florets con- 

 stituting an individual daisy. Again, when certain thistles or 

 common dandelions are fruiting, a shower of rain causes the 

 white heads of thistle-down or the " dandelion clocks " almost to 

 disappear ; this occurrence is likewise due to the bending upwards 

 and inwards of the bracts and the consequent compression and 

 concealment of the fluffy fruitlets ; these remain imprisoned until 

 the return of fair weather, when the bracts again bend outwards 

 and so allow the fruitlets to sail away on the wind. 



An inquiry into the mechanism by which these and other 

 analogous movements are effected has led to a recognition of the 

 fact that the motile organ may be constructed on radically 

 different principles in different plants ; moreover, that whilst the 

 direct intervention of the living protoplasm is responsible for 

 many of them, in other cases the mechanism is of a more purely 

 physical and less complex nature. 



The mechanisms concerned directly in producing movement 

 all depend upon alterations in the distribution of water but they 

 may be grouped conveniently in three classes as follows : — 



i. Protoplasmic. — These are related to changes in the con- 

 dition of the protoplasm or its contents in the living cell. Such 

 movements, which often occur in response to more or less 

 definite stimuli, depend in many instances on modified conditions 

 of turgescence, at any rate in those cases in which the movement 

 of a multicellular tissue is concerned ; there are other examples 

 however, which are not so readily susceptible of explanation. 



2. Hygroscopic Change.— This class of movements are caused 



454 



