504 



PLANT GROWTH AND PLANT COMMUNITIES 



Investigations of inflorescences. Evident features of many inflo- 

 rescences are their almost geometrical regularity and harmonious de- 

 velopment. The developmental concepts discussed earlier in this paper 

 seem to have an apt application to many aspects of inflorescence con- 

 struction and suggest scope for new investigations. 



Reference may be made to some observations (Wardlaw, 1960) 

 on Tussilago farfara, the common coltsfoot (Figure 10). With the onset 

 of the reproductive phase ( which begins about September, as Grainger 

 had already noted in 1939), the shoot apical meristem changes con- 

 siderably in size and form. It now gives rise to a large number of 

 bracts, instead of to a few large foliage leaves. Subsequently the pri- 

 mordia of the ray florets begin to be formed acropetally on the nascent 

 capitulum, the center of which is still bare (see Figure 11). Later, 

 however, floret primordia— destined to become the male disc florets- 

 are also formed in the central, summit region of the capitulum. These 

 are at first approximately the same size as the adjacent ray floret pri- 

 mordia, but they rapidly outgrow them. In this inflorescence the growth 

 pattern thus undergoes important changes during development. Goebel 

 (1913) also illustrated this relationship of ray and disc florets in Filago. 

 The compound inflorescence of Petasites hybridus, a species usually 

 considered to be nearly related to Tussilago farfara, shows an interest- 

 ing parallel growth phenomenon, in that the terminal capitulum be- 

 comes appreciably larger than the lateral ones, although the formative 

 sequence is acropetal. 



In Tussilago farfara and Petasites hijbridus, if a number of the still 

 developing, scale-like bracts that enclose the very young capitulum are 

 dissected off and the plant is allowed to continue its growth, the next 



Figure 10. Mature capitulum 

 of Tussilago farfara in longi- 

 tudinal median section (H), 

 showing a group of relatively 

 small disc (male) florets in 

 the center, surrounded by 

 large ray florets, also ray (fe- 

 male) (7) and disc (/) florets 

 dissected out. (After S. G. 

 Jones.) This final stage in de- 

 velopment is in marked con- 

 trast to the distribution of 

 growth in the young capitu- 

 lum and young florets, as re- 

 vealed by morphogenetic in- 

 vestigations and illustrated in 

 Figure 11. 



