28 
DR R. C. DAVIE ON 
Within the genus Spiraea (in the wide sense employed above) there is a diver- 
gence in the method of development of the segments of the leaf. Acropetal develop- 
ment is the rule in Spiraea sorbifolia and S. Lindleyana (Trecul, ’53, p. 251) and in 
S. Aruncus (Massart, ’94, pi. ii. fig. 23) ; basipetal development occurs in S. lobata 
(Trecul, ’53, p. 277). The order of development of the pinnae has not been followed 
in every species of Spiraea described above, but the four just named give us repre- 
sentatives of the groups which are sharply contrasted in type of leaf-trace. One 
form of leaf-trace is found among the species whose leaves develop acropetally, 
another in those with basipetal development. 
Professor Bower {Pliil. Trans., 1884, p. 607) has pointed out that “the arrest 
of the apical growth of the phyllopodium, and the tendency to develop the pinnae 
in a basipetal succession, progress simultaneously” in Vascular Cryptogams and 
Gymnosperms, and adds : “It can hardly be doubted that the two phenomena are 
mutually connected.” We have already seen in some of the Cycads that the tendency 
to develop the parts of the leaf in a basipetal succession has some effect 'on the 
structure of the leaf-trace. It is very probable that the differences in leaf-trace 
structure between the groups of species of Spiraea are to be related to the differences 
which we find to exist in the manner of development of their leaves. 
But there is another factor which we must consider in dealing with leaf-trace 
differences among the Dicotyledons. The process of secondary thickening in the 
Dicotyledonous . stem, has no doubt an influence on the structure of the leaf-trace. 
And the degree to which secondary thickening is developed in the stem of any 
individual plant probably affects the outline of the leaf- trace. De Candolle (’79, 
p. 433) has shown how this appears in the leaves of a single plant, from the young to 
the adult state ; he contrasts, too, the type of leaf-trace (the “ closed ” type) found in 
families with woody species with that (the “ open” type) found in families with 
herbaceous species. Col (’04) has shown, however, that the “open” leaf-trace 
system does not always occur only in herbs ; it appears, for example, in Viburnum 
Opulus (Col, loc. cit., p. 132) and in Pyrus Aucuparia (de Candolle, ’79, p. 429). 
From the list given by Eichler (’ 61 ) of plants whose leaves are developed in 
acropetal or in basipetal fashion, examples have been chosen * of herbaceous and 
woody plants. 
In Staphylea pinnata, Linn., the leaf is developed acropetally (Eichler, ’ 61 , 
p. 18 ) ; the leaf- trace is of the “ closed ” type of Spiraea Lindleyana ; the pinna-traces 
go off from the corners of the leaf- trace nearest to the pinnse, on the adaxial side. 
Basipetally-developed leaves are found in Rosa arvensis, Huds. (Trecul, ’53, 
pp. 275, 276), R. canina, Linn., and R. tomentosa, Sm. (Eichler, ’61, p. 18). The 
leaf- traces of all three are alike, open on the adaxial face ; the pinna-traces are 
formed from the margins of the leaf-trace. 
An exactly similar process appears in Potentilla Anserina, Linn., a herbaceous 
* Chiefly on account of their accessibility. 
