^*|=^'1 Ktllly, Concerning Acacia Phyllodes. I17 



Goebcl (" Organography of Plants," part ii., pp. 353-7) 

 discusses this leaf-history in an interesting manner, and refers 

 to the adaptation against loss through intense transpiration 

 in Viminaria dcnudata, and also to phyllode characteristics in 

 the Cassias. He illustrates also from the leaf features of 

 Hakca trifitycula. This genus affords an interesting field for 

 leaf investigation. Take, for instance, the leaf of the well- 

 known Cushion Flower, Hakca laurina. It is rarely, if ever, 

 spoken of as a phyllode, yet, on examination, it a})pears with 

 its straight veins as much one as any acacia. Referring to the 

 acacia, Goebel says : — " The leaf lamina is not in a condition 

 in which it could change in response to the requirements of the 

 environment, whilst the leaf-stalk, which arises, as we know, 

 at a later period in the course of the development, remains more 

 plastic. Outer conditions form a stimulus. The primordium 

 of the lamina is always present in the jihyllode, although, in 

 most cases, its capacity for development is limited only to 

 the seedling plant. In some species, such as A. alata, the lamina 

 can always be seen in a vestigial state upon the phyllode. It 

 becomes stationary and arrested." 



The earliest leaf form we have in the race-history of the 

 acacia is bipinnate. This form, we are taught, is highly 

 specialized, and is evolved from the entire form by gradations 

 through the serrate, crenulate, and lobed forms, until the great 

 surface aggregate of doulily-feathered leaves is attained. It 

 is a fair conclusion, then, that the bipinnate leaves of the A. 

 deciirrens group were derived from leaves with entire margins, 

 probably shaped like the broader phyllodes found on other 

 sj)ecies to-day. Were it not that we find now on those 

 jihyllode-bearing species seedling and obsolescent pinnate 

 leaves, it would l)e a natural conclusion that the dccurrcns 

 grouj) was the farther advanced in foliage evolution — at least, 

 so far as its morphology is concerned. This would be true, 

 too, if the phyllode were not only a regression but also a 

 degeneration. The phyllode, however, is an adajitation to 

 environment, and consequently must be progressive, yet it is 

 ])robably also a regressive form, whose recajiitulation is 

 embodied in the cotyledons of the embryo ; but it is also jiro- 

 gressive. 



Let me draw an illustration from i)olitics. A. is jH-emier 

 to-day. In his absence B., one of his ministers, is acting- 

 premier. While these circumstances exist, A. is the real 

 premier. Later on A. retires altogether, and B. succeeds him. 

 B. is then the real premier. A.'s iMemiershi]) is only a fact 

 of past history. As soon as one form of foliage is comjiletely 

 superseded by another, the latter takes the place and performs 

 the functions of the former leaf. This new form is the true 



