CORRELATIONS 339 



directly on the growing parts, at all events on the growing point of the shoot ; 

 light, heat, chemical and physical influences of the material environment never 

 affect the growing point itself. At the growing point or in its immediate neigh- 

 bourhood it is decided what the organ shall develop into, and although external 

 factors take part in this determination they can only do so indirectly through 

 neighbouring older parts, that is, correlations always play a part. 



Whether, however, the external factors operate directly on the part in pro- 

 cess of formation or through the agency of older parts, they are always of the 

 nature of releasing energies, and not the actual causes of the formation itself ; 

 that must lie in the plant itself, or more exactly in the protoplasm. Hence it 

 is that plants have a cell structure and develop growing points, leaves, buds, 

 roots, &c. ; protoplasm is also responsible for the differences in the organs men- 

 tioned in different species. Although we may hesitate whether to consider the 

 correlations as due to internal formative factors or not, we know sufficiently 

 accurately that all organic formation in which protoplasm is concerned is condi- 

 tioned by internal factors. In this domain lie the genuine problems of develop- 

 mental physiology, from any insight into which we are as yet entirely excluded. 

 It is as well to note that although we may establish the existence of one of the 

 external factors in development we no more thereby attain an insight into the 

 subject than we do into the structure of a complicated steam-engine by know- 

 ing who opened the valve. Since we learn nothing as to the mode of working 

 of the machine by knowledge of this kind, it cannot be correct to designate the 

 modest beginnings of developmental physiology as ' developmental mechanics '. 



A short time since G. KLEBS (1903) classified the factors in plant form in 

 three categories ; he distinguished external and internal conditions of the re- 

 sulting form and separated from the latter ' specific structure ', what we have 

 designated as 'those in which protoplasm is concerned'. Specific structure is 

 to a certain extent constant for each organism and on that depends everything 

 else. The external world never acts on the specific structure directly but 

 always on the internal conditions, on the quantity and quality of the substances 

 present in the cell, and on the physical characters of the protoplasm, vacuole, 

 cell- wall, &c. ; and these internal conditions in turn affect specific structure. 

 The internal conditions are to a certain degree open to investigation but the 

 results of their actions are open only to a limited degree. 



Although KLEBS' s conclusions are, in principle, convincing, we still en- 

 counter difficulties in determining in special cases what depends on internal 

 conditions and what on specific structure, and we have in the lectures which 

 immediately follow, which were written before KLEBS' s paper was published, 

 distinguished only between internal and external factors ; in separating these 

 we make use of a sufficiently sharp but certainly superficial criterion. The 

 external factors are such as may be referred to the action of gravity, light, &c., 

 while the internal factors are those whose direct dependence on the external 

 world cannot be demonstrated. 



Bibliography to Lecture XXVI. 



ARNOLDI. 1900. Flora, 87, 440. 



BERTHOLD. 1882. Jahrb. 1 wiss. Bot. 13, 569. 



{BERTHOLD. 1904. Z. Phys. d. pflanzl. Organisation. Leipzig.] 



[FALKENBERG. 1901. Die Rhodomelaceen d. Golfs v. Neapel. Berlin.] 



GOEBEL. 1880. Bot. Ztg. 38. 



GOEBEL. 1884. Die gegenseit. Bez. d. Pflanzenorgane. Berlin. 



GOEBEL. 1893-95. Flora, 77, 38; 81, 195. 



GOEBEL. 1898-1901. Organographie. Jena. 



GOEBEL. 1903. Biol. Centrbl. 22, 385. 



[GOEBEL. 1905. Flora, 95, 384.] 



Z 2 



