30 INTRODUCTION 



Although, therefore, in a particular case a perianth segment may be 

 regarded on phylogenetic grounds as a modified leaf, nevertheless the 

 actual development of the primary meristem tissue may be such as to directly 

 lead to the formation of the perianth segment, without, even transitorily, any 

 of the vestigial inductive conditions coming into play, such as formerly led 

 to the formation of a leaf. Hence it is just as possible that the inductive 

 influences which act upon the rudimentary organ are, at first, such as would 

 lead to the formation of a floral organ, while a secondary change in the 

 nature of the guiding influences at an early stage of development leads to 

 the production of a foliage leaf, as that the reverse process takes place. 

 It is perhaps simpler to regard the early developmental stages as being 

 similar in both cases, the subsequent divergence to form either a foliar 

 or a floral organ being determined by the external and internal influences 

 which act upon the meristematic rudiment. 



The correlative and other phenomena just mentioned are merely special 

 examples of the regulatory processes by which the vital mechanism of the 

 entire plant is linked together and adjusted. Without such purposeful 

 regulatory changes the progress of development along particular lines, and 

 according to definite laws, would be impossible ; nor could a harmonious co- 

 operation of the component parts be attained, in the event of a compensatory 

 alteration becoming necessary in correspondence with a change in the 

 external conditions. 



Self- regulation is attained in machines as well as in plants by allowing 

 the character and amount of the work done, or the rate at which it is 

 performed, to influence the working mechanism. The same general principle 

 is involved, whether the regulatory mechanism is intended to maintain 

 a condition of equilibrium, or whether progressive or periodically recurring 

 alterations are to be compensated by a corresponding accommodation 

 of the working mechanism to the changed conditions. The governor of 

 a marine engine affords a good example of how the working activity 

 of a machine may bring regulatory processes into play, which tend to 

 maintain a condition of working equilibrium. Speaking in general terms, 

 the needs of a self-regulating mechanism act as the causes inducing 

 a response to the demand. Thus transit of nutriment is regulated by 

 consumption, a tendency to perfect the organism induces isolated shoots to 

 form roots, and the demands for further supporting power may increase the 

 tensile strength of a branch. At the same time these examples show that 

 under normal conditions the potential powers are not exercised to the utmost 

 possible extent, as must, indeed, always be the case when any increased 

 activity may be induced. 



The causative relationships of regulatory processes are, in most cases, 

 still rather obscure. It is certain, however, that the very varied regulatory 

 processes which are universally present may be maintained by various means 



