214 INFLUENCE OF CORRELATION AND EXTERNAL STIMULI 



lacustris I have described 1 , in which in definite circumstances the forma- 

 tion of sporangia is suppressed, may be cited here, likewise the case of 

 the old prothalli of Doodia caudata upon which the sexual organs arc 

 abnormal and apogamous shoots are developed ; probably in other cases 

 in which these appearances are seen the process is the same. 



2. QUALITATIVE INFLUENCE OF CORRELATION. 



The relationships of direction due to the qualitative influence of cor- 

 relation must next be referred to. Such relationships are conditioned 

 primarily by reaction to outer stimuli, such as geotropism and others, but 

 correlation also is an important factor. This is specially the case where 

 a plagiotropous lateral shoot springs out of an orthotropous chief axis. 



The relationships are seen most simply in the roots. When speaking 

 of regeneration I stated that the severance of the tip of the chief 

 root influenced in many cases the growth of the lateral roots ; that 

 lateral root which stands next the cut surface is devoted to the elongation 

 of the primary root and takes on therewith its peculiarities, that is to 

 say, capacity for growth and branching becomes increased and the 

 former lateral root becomes the foundation for the further differentiation 

 of the root-system. 



The conifers supply the most instructive examples for the shoot- 

 system. The dorsiventral construction of the lateral branch in the spruce 

 has been depicted above, but this is only brought about by its plagiotropic 

 growth which is a consequence of correlation. If one cuts off the top 

 of the main shoot, the nearest lateral shoot 2 , always supposing that it 

 is not too old, grows erect and takes on a radial construction quite 

 like that which is characteristic of the chief shoot. We must recollect 

 however that the dorsiventrality induced in the side-shoots through their 

 plagiotropous growth is to a certain extent differently fixed in the several 

 forms. In the firs this substitution of a lateral shoot for the terminal 

 one is less easy than in the spruce ; it would appear that it takes place 

 better the stronger the plant and the younger the shoots. Should a side- 

 shoot not become erect, one or more radial shoots issue from its base 

 and one of them takes the place of the terminal twig. Araucaria does 

 not possess this capacity of transforming a dorsiventral lateral shoot 

 into a main terminal one ; in this genus the lateral shoots are branched 

 only in one plane in by far the greater number of cases, and so far 

 as we at present know this peculiarity appears to be fixed in them 

 from the beginning. It is of course possible that in Araucaria. in accord- 

 ance with what has been already said about Phyllanthus 3 , the lateral 



1 Goebel, Uber Sprossbildung auf Isoetesblattern, in Botan. Zeitung, 1879, p. i. 



2 Or it may be mnny lateral shoots. 3 See p. 97. 



