president's address. 29 



lu seedliugs ul' ,i. disculur, one of the very eomuiuu l)ipmuate Acacias of the 

 Sydney district, for example, the first leaf is abruptly pinnate with about six 

 pairs ot leaflets; tlie second, third, and fourth may be bipinnate with seven pairs 

 of leaflets on the second and third, and twelve pairs on the fourth. The iifth, 

 sixth, and seventh may be bipinnate, with two pairs of pinnre ; these correspond 

 to a leaf like the first, in which the apical pair of leaflets, and the pair next 

 below, have been replaced by pairs of pinna'. The eighth leaf may have three 

 pairs of piuna;; this corresponds to a leaf like the fiist, in which the apical pair 

 of leaflets, and two pairs next below, have been replaced by pinnae. After the 

 eighth the number ot pinnae may increase by one pair more or less consecutively 

 in succeeding leaves, until something approaching the maximum is attained. In 

 one seedling however, and the only one seen, the third leaf had two pairs of 

 pinnffi. In seedliug-s of other species, the number of pairs of pinnae increases 

 sometimes a little sooner, sometimes a little later, much in the same manner as 

 described above in A. discolor. The terminal seta, unless accidentally missing, 

 terminates the common petiole of every leaf, at every stage of growth. There- 

 fore, the mode of succession of the pairs of pinnae in the gradual development 

 of the bipinnate leaf is basipetal, and not basifugal, as has hitherto been tacitly 

 assumed and taken for granted in every case that has come under my notice. 



The primary leaf-axis of the tii-st, abruptly pinnate leaf may be slightly 

 longer than that of the second bipinnate leaf with one pair < f pinnae, but the 

 latter have uiore than twice as many leaflets. As the number of pinnae increases, 

 the axis lengthens proportionally, until it reaches its final dimensions. When 

 the maximum number of pairs have been developed it will be noticed that the 

 petiole is relatively short. 



Young seedlings of the Euphyllodineie, old enovtgh to show the transition 

 from bijjinnate leaves to euphyllodes, are very interesting and insti'uctive. They 

 are the embodiment, and, at the same time, the visible presentment or picture of 

 an intense struggle between two antagonistic tendencies or forces. On the one 

 hand, the hereditary tendency to produce the ancestral type of foliage makes a 

 start in the normal way. The first leaf is an abruptly pinnate leaf, or, in some 

 species, there may be an opposite pair of them. The second leaf is an abruptly 

 bipinnate leaf with one pair of piiinae, just as in the seedlings of the Bipinnatfe. 

 Very soon, somewhat sooner in some species than in others, the antagttnistic ten- 

 dency, the euphyllode-producing tendency, nowadays also an inherited tendency, 

 manifests itself, and, after a few preliminary stages, the usurper succeeds in 

 swamijing tlie natural tendency to continue tlie production of bipinnate foliage. 

 This commonly, but not always, happens before the seedlings are strong enough 

 to enable the biijinnate leaves to develop a second pair of pinnte; and, in such 

 species, the second, third, fourth, fifth leaf or some later one may be the earliest 

 complete euphyllode . 



The ol)ject of the struggle is to get rid of the pinnae, whose leaflets are the 

 transpiring and assimilating organs proper, and to substitute for them the ver- 

 tical, flattened, leaf-like leaf-axes or common petioles, or euphyllodes, capable of 

 taking over and carrying out the functions of the leaflets on a more economical 

 basis for regulating the water-supply and expenditure. It is important to realise 

 this; and that the contest is not between flattening, ordinary petioles, and leaf- 

 Wades, which are to vanish. Now a substitutional structure, and a structure for 

 which a substitute is being provided, cannot comjjletely coexist and function in 

 all respects simultaneously. In the case of the substitution of euphyllodes or 



