550 TRANSFORMATION OF ENERGY 



arises whether it is the direction or the intensity of the light that has the 

 greater influence. There can be no doubt that, in general, phototactic move- 

 ments are carried out in a state of nature so that swarmspores place their 

 long axes parallel with the incident beam, and further we have every reason to 

 believe that the plant aims not at orientating itself in a definite direction to the 

 path of the rays but at placing itself under an optimum light intensity. The 

 only question is whether it is possible that experimental conditions may be 

 arranged under which no light rays pass from the brighter to the darker region 

 of the apparatus in which the experiment is conducted. According to Oltmanns 

 it is possible (Fig. 170) to arrange that parallel rays of sunlight may fall at right 

 angles on the lateral wall of a glass trough in which the swarmspores are moving ; 

 immediately in front of the wall exposed to the light he placed a prism filled with 

 gelatine in which indian ink had been dissolved. Under these circumstances 

 the light rays will fall on the glass trough parallel to each other (as indicated by 

 the arrows), but their intensity will gradually decrease from one end of the trough 

 to the other. If now phototactic organisms are uniformly distributed in the 

 water in the vessel they will all collect together on the illuminated wall only, 

 and there also distribute themselves uniformly. If they seek, however, 

 a definite intensity of illumination they must obviously move at right angles 

 to the direction of the incident ray. The result of this experiment is that we 



invariably find an aggregation of swarmspores 



_ at a point where the light is of definite in- 



tensity, viz. the optimum. Several criticisms 



may be advanced, which tend to shake our 



confidence in this experiment ; first of all, 



_ Oltmanns has not arranged that the sunlight 



should fall horizontally and at right angles to 



the prism, but has allowed it to fall on the 



Fig. .70. Plan of ot-TMANNs- experi- darkened side wall in its natural direction, and 



mental apparatus. Gi, vessel containing hcuce the distribution of light intensity and 



:'rhrd?a'n''inr'Tt'=arSws Ete'lhe the direction of the rays in the culture vessel 



direction in which the light rays fall on the haVC bcCU SOmCWhat OVCrlookcd ; but CVCU if 

 veisel, and their size indicates the intensity , , • , , ■ ^ , • ,-, j 



of the light. the experiment be earned out m the way de- 



scribed all difficulties would not thereby be 

 removed. If the light-absorbing prism be quite homogeneous, and if there be 

 an empty space behind it, then certainly our supposition as to the direction of 

 the rays and the distribution of the light is correct, but in the prism itself, in 

 the glass, and finally in the water in which the organisms are distributed as 

 well, we always find reflection of light, and hence the experiment becomes 

 perfectly useless for the purpose intended (Towle, 1901) (compare p. 472). 



There are not only physical difficulties to be considered but physiological 

 difiiculties as well. Rothert's (1901) observations on strophic and apobatic 

 tactic movements apply naturally not to chemotaxis alone but to all forms of 

 taxis. Apobatic phototaxis we have long been acquainted with ; Engelmann's 

 (1882) experiments with Euglena are in the highest degree valuable, for they 

 leave no doubt in our minds that these organisms recoil when passing from light 

 into darkness. An isolated spot of light with a dark surrounding operates at 

 once upon them ; other and earlier experiments carried out by Cohn (1852) 

 and Famintzin (1867), and confirmed by Strasburger (1878), are unintelligible 

 without the assumption of apobatic phototaxis. In these experiments £Mg/^Wfl, 

 Stephanosphaera, Haematococcus, &c., were placed in shallow dishes which were 

 exposed to direct sunlight, and which were therefore illuminated uniformly ; 

 if a narrow plate is then laid transversely across the vessel the motile organisms 

 rapidly assemble in the half-shaded places, leaving the regions of greatest 

 shade and also those on which the direct sunlight falls. It is quite impossible 



