ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
65 
Leaves of Conifers.* — M. A. Daguillon draws the following conclu- 
sions from his work on the evolution of the leaves of the Abietineae : — 
(1) The existence of primordial leaves, that is, of leaves intermediate 
between the cotyledonary leaves and those of the adult plant, is con- 
stant. (2) The passage from the primordial form can take place without 
numerous transitions, as in Pinus , or by insensible gradations, as in 
Abies. (3) This passage is sometimes characterized by a modification 
in the phyllotaxis. (4) Sometimes also it is marked by a change in the 
state of the epidermal surface. (5) It is nearly always accompanied by 
the development, below the epiderm, of one or more sclerenchymatous 
layers, which afford the leaf protection and support. (6) The pericyclic 
sclerenchyme, which incloses more or less completely the median vein, 
acquires a considerable development. Further, among the two sorts of 
elements of which it is composed (cells with areolated punctations and. 
fibres with smooth membranes), the latter are often absent from the 
primordial leaves, appearing with the passage from the primordial to 
the definite form. (7) In certain genera (Abies, Pinus) the fibro vascular 
system of the median vein proceeding from a single bundle of the stem 
bifurcates in the interior of the adult, while it remains simple in the 
primordial leaf. (8) In all cases the number of the conducting elements 
of the xylem and of the phloem augments when the primordial passes to 
the mature leaf. (9) When the foliar parenchyme is heterogeneous and 
bifacial, the differentiation of the palisade-tissue is generally accen- 
tuated in the adult leaves. 
Leaves of Marine Phanerogams.! — Pursuing his examination 
of the leaves of aquatic plants, M. C. Sauvageau states that the family 
of Hydrocharidese contains only three genera adapted to live in sea-water, 
Enhalus, Thalassia, and HalopJiila. The leaf of Enhalus aceroides , be- 
sides its dimensions and absence of ligule, is distinguished from that of 
all other marine flowering plants by the long fibrous filaments, by the 
anatomy of the fibro vascular bundles, and by the double orientation 
of the fibrovascular bundles of the limb. TJialassia differs from Ew- 
halus in the structure of the limb, the two species of TJialassia differing 
from one another in the nature of the teeth at the extremity of that 
organ. In HalojpJiila there is but very slight differentiation in the 
structure of the leaf. 
The small genera Halodule and Phyllospadix present nothing very 
remarkable in the structure of their leaves. Halodule has secreting 
cells which are entirely epidermal ; both genera have non-lignified 
fibres in the vascular bundles between the xylem and the phloem. 
Summing up the conclusions of his study of the leaves of marine 
Phanerogams, M. Sauvageau states that if those flowering plants which 
live normally in the submerged condition are descended from terrestrial 
plants which have adapted themselves to this new mode of existence, 
the adaptation must have taken place in several different ways. The 
presence and the importance of a more or less lignified mechanical system 
vary greatly in the different genera. Except in the genus Halophila , 
* Eev. Gen. de Bot. (Bonnier), ii. (1890) pp. 154-61, 201-16, 245-75, 307-20, 
345-58 (4 pis. and 68 figs.). 
f Journ. de Bot. (Morot), iv. (1890) pp. 269 73, 289-95, 321-32 (12 figs ). Cf. 
this Journal, 1890, p. 741. 
1891. 
F 
