154 PLANT PHYSIOLOGY 



with those animal movements which, though they take place through the 

 agency of nerves and a central nervous system, yet awake no consciousness, 

 viz. with so-called ' reflex actions ', and, as a matter of fact, this comparison 

 has often been made (OLTMANNS, 1892 ; CZAPEK, 1898). Indeed the charac- 

 teristic feature of a reflex action, the transformation in the central organ of 

 a sensory excitation transmitted to it into a motor excitation, is doubtless 

 entirely absent in the plant, where any central organ comparable with a gang- 

 lion cell is non-existent (comp. FITTING, 1906, p. 239). Although we deny 

 the occurrence of genuine reflexes in the plant, still we must all the more 

 emphasize the existence and very wide distribution of the more simple forms 

 of sensory conductivity. 



After directing attention to FITTING'S admirable exposition of the pheno- 

 mena of stimulus conduction and to the statements on the subject made already 

 in many places, we need only say in a word that there are obviously two types 

 of stimulus conduction in the vegetable kingdom, neither of which ought to 

 be compared with conduction in an animal nerve ; the one type is that which 

 takes place along the course of a vascular bundle for long reaches and with 

 conspicuous rapidity ; in that case we have doubtless to do with a compara- 

 tively rough mechanical conduction. On the other hand, the stimulus in other 

 cases is propagated from cell to cell in all directions in ordinary parenchyma. 

 Perhaps, if the reaction consists of relatively simple phenomena, a chemical 

 alteration may be transferred through the tissues, and originate an excitation 

 in the protoplasm of every part that it reaches. If a tropistic curvature is the 

 result of the stimulation, a ' polarized state ', about which nothing is really 

 known, must be transmitted from cell to cell from the perceptive organ where 

 it originated. 



A glance at these cases of stimulus conduction, as generally at the last- 

 mentioned phenomenon of stimulation, shows how simple the mechanism 

 which we employed as an illustration is as compared with the living plant. In 

 the latter there exists a whole chain of releasing actions between the first 

 application of the stimulus and the final visible response. 



526, 1. 4, for 76 read 68 



11. 29-35, for also 'optima' . . . temperature read there are also processes in 

 which well-marked minima, optima, and maxima may be recognized (comp. 

 ERRERA, 1896). 



11. 41-5, for appearance . . . true of others, read appearance in a unit of 

 time at that than at other temperatures. It is certainly questionable whether 

 phenomena of this sort bear more than a superficial resemblance to the depend- 

 ence of the organism on the outer world. It appears to us more probable 

 that the optimum curves which have so often been found to express this 

 relationship to external factors arise generally in the way which has been 

 discussed under the curve of assimilation, where it was shown that the optimum 

 curve was the resultant of two curves, one expressing the injurious action and 

 the other the stimulatory action of the same factor on the organism. 



If then in a single process, such as assimilation, temperature plays a double 

 part, we cannot wonder that the curves for other processes dependent on 

 temperature, e. g. of respiration or growth, are quite different in appearance 

 from the assimilation curve, and we cannot doubt that the same is true of 

 other formal conditions also. 



527, 11. 23-30, delete An example . . . occurrence. 



528, 11. 8, 19, 33, 38, 39, for nyctitropic read nyctinastic 



11. 40-7, for (minimum . . . petiole, read become so rapid that they may 

 be readily followed with the naked eye. At 30-35 C. a to-and-fro oscillation 

 is completed in half a minute. The rate of movement above and below this 



