I 4 2 PLANT PHYSIOLOGY 



of the stimulus to the darkened base took place with scarcely diminished 

 rapidity and in a perfectly normal manner, when the continuity of the tissues 

 was interrupted by incisions made at adjacent regions of the cotyledon from 

 opposite sides, and reaching beyond the middle. It may be deduced from 

 this that the stimulus can be transmitted transversely as easily as longitudinally, 

 and one can only conclude, as FITTING does, that a polarization of the cells 

 or of the constituents of the cells of the perceptive organ arises from the imping- 

 ing light, a polarization which is then continued downwards to all other cells. 

 We are, however, entirely ignorant as to the nature of this polarization, whether 

 it consists in the arrangement of definite protoplasmic particles, or whether it 

 lies in the accumulation of certain chemical substances which must possess 

 different characters at the two poles created by the light rays. At all 

 events from all the experiments which have been carried out there is evidence 

 showing that the response in the darkened base of the cotyledon is not a con- 

 sequence of excitation transmitted straight downwards from two unequally 

 excited parts of the apex, being differential on the concave and convex sides. 



A consideration of MASSART'S experiments (1888) has led one to a similar 

 conception. This investigator has illuminated unequally opposite sides of 

 the sporangiophores of Phycomyces, and has determined what difference in 

 light intensity induces curvature towards the more illuminated side. Cur- 

 vatures took place when the light intensities differed in the proportion of 

 100 to 118. This ratio was found to be constant for light of varying intensity. 

 Thus MASSART was enabled to prove in the case of heliotropism the validity 

 of WEBER'S Law as to the relation subsisting between the amount of the 

 stimulus and sensitivity, a law which we shall have to refer to later on in 

 reference to other stimulus phenomena, and thus he was able to confirm an 

 earlier suggestion made by PFEFFER (1884). Further investigations are 

 urgently needed, however, in order to show whether this relation is really true 

 of all light intensities ; this, we think, is scarcely likely. Be it as it may, 

 MASSART'S experiment is not to be compared with an ordinary heliotropic 

 experiment, for here we are dealing with two antagonistic stimuli acting on 

 the plant, as in intermittent geotropic stimulation, and it is able to respond 

 to the stronger of these only if the difference between them is appreciable to 

 the plant. In accordance with what has been said, we must not, on the con- 

 trary, look for the cause of the stimulus in ordinary unilateral stimulation in 

 the difference between the illumination of the side facing the source of light 

 and that facing away from it ; in this case we have to do only with a single 

 stimulus. 



To HABERLANDT (1905) we owe a new conception as to the stimulus 

 impact in heliotropic curvature. The epidermal cells of many leaves bear 

 conical papillae, which split up the impinging light like lenses, so that a median 

 region of the inner walls of the epidermal cells is more brightly illuminated 

 than the margin. This difference in illumination must, according to HABER- 

 LANDT, induce a heliotropic stimulus, the leaf being in the rest position when 

 the spot of light lies in the centre of the cell, and a movement being induced 

 when the spot is eccentric. It is not altogether easy to apply this hypothesis 

 to ortho tropic organs, and further, the theory has no sense in it, for orthotropic 

 organs which are most sensitive to light, such as grass seedlings, have no 

 papillose epidermis. Again, FITTING has shown sufficiently clearly in the 

 experiments quoted above that the inner side of the epidermis of the cotyledon, 

 or any layer of epidermal cells which may be exposed by wounding, plays 

 exactly the same part in the reception of the heliotropic stimulus as the 

 external epidermis under normal circumstances. It has yet to be proved that 

 the upper epidermis of a foliage-leaf has any determinative significance in this 

 relation ; and it is well known that not every leaf sensitive to light is papillose. 



