240 PLANT PHYSIOLOGY 



of adjacent objects, the mind is conscious of the contact. 

 Solid object and soUd object exert on one another certain 

 influences. The finger imparts a certain amount of 

 warmth, of moisture perhaps, possibly of other forces and 

 substances, to the rod, and the rod also acts upon the 

 finger. The contact of solid objects with plants and plant- 

 parts enables them directly to influence each other. Only 

 when the plant or plant-part is more responsive than the 

 lifeless object, only where there is a visible reaction to the 

 influence thus exerted, can we teU either the character or 

 the extent of the influence. By studying the reactions to 

 the influence exerted by contact we can divine something re- 

 garding the nature of the stimulus. 



When zoospores and other motile and floating spores of 

 sessile plants come to rest, attaching themselves to the sub- 

 stratum, the attachment is effected through means not yet 

 wholly clear. The course of events is briefly this. When a 

 spore, e. g. of Vaucheria, CEdogonium, Fucus, etc., comes to 

 rest against the surface of a solid object — a dead leaf or 

 branch or stone — its form changes and the naked mass of 

 protoplasm invests itself with a cell-waU. The part of the 

 cell against the solid object flattens, spreads out somewhat, 

 conforms accurately to the surface of the object, and adheres 

 closely to it. In this way the holdfast of the new plant is 

 begun. The cause of this local and peculiar growth cannot 

 be due to the stimulation of gravitation, for such spores 

 attach themselves as frequently to oblique and vertical as 

 to horizontal surfaces. Nor can it be warmth or moisture, 

 or even chemical stimulation. It must be either light or 

 contact or both. Light or darkness must play a part in 

 controlling the local growth of holdfasts in many plants. 

 It appears, however, to influence locomotion and the direc- 

 tion of division of the spores* rather than to stimulate 

 growth in the parts touching the solid object. We may 

 conclude, then, that contact with the solid stimulates the 

 protoplasm, but it remains for experiment actually to show 

 that the increased rate and the changed direction of growth 



* Winkler. H. Einfluss ausserer Factoren auf die Theilung der Eier von 

 Cystosira barbata. Ber. d. D. Bot. G«8., Bd. 18, 1900. 



