510 PLANT GROWTH AND PLANT COMMUNITIES 



there are interesting correlative changes in other floral members— e.g., 

 the calyx and corolla. 



By applying auxin exogenously to plants at precisely the right 

 stage in their ontogenetic development ( i.e., at a sufficiently early stage 

 in floral morphogenesis), he has shown that profound changes in the 

 structure and function of floral members can be induced. In species 

 with hermaphrodite ( or monoclinous ) flowers, the corolla and androe- 

 cium may be suppressed to a more or less marked extent, whilst the 

 calyx and gynoecium become relatively enlarged; in monoecious species 

 such as the cucurbits, the appearance of the first female flower may be 

 advanced in time, and the ratio of male to female flowers falls. Again, 

 in a dioecious species such as the hemp plant (Cannabis sativa), male 

 plants produce female or intersexual flowers. It is important to empha- 

 size that the auxin in these instances appears to be acting as a regulat- 

 ing rather than as a primary determining factor: i.e., its effects are 

 restricted to changing the balance of growth between floral organs of 

 different kinds. This is in accord with the writer's theory (Wardlaw, 

 1957c ) that the basic properties of the growth centers of any particular 

 whorl or helix on the receptacle (e.g., of calyx, corolla, etc.) are al- 

 ready determined as the result of serial genie evocation. 



In short, specific morphogenetic developments can be suppressed 

 but usually not fundamentally modified, though instances of organs of 

 intermediate or dual character are not unknown. As J. Heslop-Harrison 

 has noted: "There is no recorded instance of a primordium of one 

 prospective type being deflected into a foreign developmental path as 

 a result of auxin treatment." In dioecious hemp, however, the auxin- 

 induced intersexuality and sex reversal in genetically male plants 

 "seems to arise from a diversion of the ontogeny of the presumptive 

 stamens from their normal path towards the characteristic of carpels"; 

 that is to say, "auxin may be influencing some determining process in 

 the flower primordium." These effects of applied auxin are, of course, 

 to be linked with the auxin changes now known to occur in plants 

 variously exposed to different environmental factors. 



Certain fungal infections are known to cause remarkable changes 

 in floral development: e.g., when Zea mais is infected with Ustilago 

 maydis or Melandrium rubrum with Ustilago violacea. In the former, 

 female flowers appear in the normal male inflorescence and J. Heslop- 

 Harrison (1959) notes that this effect is now understandable if the 

 monoecism of maize is related to an auxin gradient, in that the fungus 

 is known to produce I AA in the presence of tryptophane ( Wolf, 1952 ) . 

 In Melandrium rubrum, the pathogen induces the formation of stamens 

 (see Figure 14) in genetically female plants (Schopfer, 1940; Baker, 

 1947, a, b). These are only some examples of the effects of extrinsic 

 factors in floral morphogenesis ( see the literature in J. Heslop-Harrison, 

 1959). 



