CAUSAL RELATIONSHIPS OF GROWTH AND DEVELOPMENT 25 



pelled to reproduce either entirely asexually or entirely sexually, according 

 to the conditions under which they are cultivated. In such plants, unless the 

 appropriate variations of the external conditions took place, no alternation 

 of sexual and asexual generations would occur. On the other hand, no 

 permanent periodicity could be induced in organisms of this character, even 

 though they encountered an indefinitely prolonged continuance of the 

 alternating changes in the external conditions which induce periodicity, such 

 as the succession of day and night, or the regular alternation of the seasons. 



In nature the external conditions are never constant, nor can any 

 precise uniformity be artificially maintained for any length of time. 

 Nevertheless there can be no doubt that very many, and perhaps most 

 plants, would continue to exist and perpetuate their kind under perfectly 

 constant external conditions. For the continued existence of other plants, 

 however, variations in the internal and external conditions may be absolutely 

 essential. Strictly speaking, this is actually the case in those hetercecious 

 parasites which are able to reach their full development, and complete their 

 life-cycle only by migrating from one host to another. 



As the result of antagonistic or mutualistic interactions, heteromorphous, 

 or, in other words, formative (morphogenetic), changes of remarkable 

 character are often produced in response to the action of stimuli. The 

 formation of insect or fungal galls by plants forms a good example of 

 such modifications of growth, while in Euphorbia Cyparissias the branches 

 assume a very special and peculiar shape when attacked by a certain 

 aecidium. Lichens afford especially instructive examples of the shape 

 of a compound organism remaining constant, so long as the interacting 

 factors which produce it are unaltered. 



It has already been pointed out that, with the exception of those 

 cases in which growth is directly and mechanically restricted as, for 

 example, when a root is grown in a plaster cast, or penetrates an aperture 

 of given shape the external influences do not directly give rise to changes 

 in shape and structure, but are merely the agencies which initiate that 

 modification of the vital activity of the plant to which the alteration is due. 

 When such phenomena are indicated by terms derived from the external 

 agencies which induce them (Photomorphosis, Chemomorphosis, Bary- 

 morphosis 1 ), we are no nearer a complete understanding of the chain of 

 internal processes which leads to the final result, than we are when we say 

 that a stem bends towards the light because of its heliotropism. 



Among these extremely complicated internal reactions, the counter- 

 influences of different organs or cells of the plant play a most important 

 part. These influences originate in the plant itself, but nevertheless, so 

 far as the part affected is concerned, they must be regarded as component 



1 Sachs, Flora, 1894, p. 231 ; Herbst, Biol. Centralbl., 1895, Bd. XV, p. 



