514 PLANT GROWTH AND PLANT COMMUNITIES 



may be present at some stage in the ontogenesis of many flowers may 

 also be noted (see Cutter, 1957). In comparative ontogenetic studies 

 of the flowers in species of Nymphaea and Nuphar, Cutter observed 

 that the "subtending" bract in the latter, which is the first primordium 

 to originate on the floral meristem, is homologous with the anterior 

 sepal in Nymphaea, which has no bract. In both instances the position 

 of the first floral organ suggests that there is some difiFerence in growth 

 between the anterior and posterior sides of the young floral meristem, 

 but on further development, radial symmetry is established. If, how- 

 ever, we suppose that the incipient growth difference between the 

 anterior and the posterior (or abaxial and adaxial) sides of the flrxal 

 primordium were to be accentuated, then the flower would be zygo- 

 morphic. This kind of relationship can perhaps be examined experi- 

 mentally. 



In a species in which the flower is normally actinomorphic, it may 

 be assumed that the growth on the adaxial and on the abaxial faces is 

 approximately equal in amount: i.e., such controlling or regulating 

 effects as may be exercized by the axis, or inflorescence apex, above are 

 equalled by those exercized by the bract below. If, now, the very young 

 floral primordium were to be isolated from the controlling effects 

 from above by a tangential incision, some degree of zygomorphy in the 

 developing flower might be expected to follow. In fact, when a flower 

 locus was isolated from the inflorescence apex in Primula bulleyana by 

 a tangential incision, Cusick (1959) found that a flower duly devel- 

 oped, and the present writer notes that it seems to show some zygo- 

 morphic development. 



Another approach to the phenomenon of zygomorphy is perhaps 

 afforded by the classical case of meristic variation in Stellaria media, 

 especially in the androecium, varying from three to eight stamens 

 (Figure 16). Matzke (1932, 1952) has shown that the distribution of 

 missing stamens is not entirely at random but is such as to approximate 

 an incipient dorsiventrality between the axis and the bract. It would 

 be interesting to know whether other instances of seemingly random 

 meristic variation are also of this kind. In the extreme form of andro- 

 ecial meiomery seen in Orchidaceae and Zingiberaceae, it is of interest 

 to note that whereas the residual stamen of the former is in the anterior 

 position, that of the latter is in the posterior position, both flowers 

 showing median zygomorphic symmetry ( see Figure 15 ) . 



However conspicuous the departures from radial symmetry may 

 be, floral development typically shows evidence of an over-all balance 

 and stability. This is what one would expect on the assumption that the 

 floral meristem is a unified reaction system, producing new organs in 

 a characteristic pattern at intervals in accord with the laws of physical 

 chemistry, and with pervasive reciprocal relationships between the 

 younger and the older organs throughout the floral ontogenesis. 



