514 
many plants which similarly press close toa flat surface. 
In Veronica hederefolia we get it in a weed that climbs 
over banks of earth ; in Zizaria cymbalaria we get it in 
a trailer hanging upon stone walls; in Campanula hede- 
racea and Ranunculus hederaceus we get it in a creeper 
along the edge of rills or over soft mud. Compare in 
each case other forms of the typical generic leaf, as seen 
in germander speedwell, toadflax, harebell, and meadow 
buttercup. 
Another special climbing type, proper to more open 
habits of twining round alien stems, is that of the com- 
mon bindweed. This, the ordinary convolvulus form, 
reappears exactly in so distant a plant as Polygonum con- 
volvulus, whose habits are exactly similar. Even among 
monocotyledons we get it closely simulated by Smz/ax, 
with precisely like conditions, and somewhat less closely 
by Zamus. Indeed, this form of leaf may be said to be 
almost universal among lithe twining creepers. 
The hop type belongs rather to mantling than to mere 
twining climbers, It reappears under identical condi- 
tions in the vine, and less closely in true bryony. More 
subdivided into leaflets, it produces the Virginia creeper, 
and many forms of clematis. 
Among ground plants, it is only possible very briefly to 
refer to the succulent types which abound in dry situa- 
tions. A regular gradation may here be traced from 
rich forms with rather thin, flat, ovate leaves, growing in 
favourable situations, like Sedum telephium, through 
dwarfish forms, with oblong leaves, like Sedum album, to 
forms with knobby, globular leaves, growing in very dry 
spots, like Sedum anglicum. Where the stem becomes 
very succulent, the leaves may be dwarfed out of exist- 
ence altogether, or reduced to prickles, as in those dry 
desert plants, the cactuses. Compare some tropical 
Euphorbias. Miscellaneous examples of these dry types 
are also found among Mesembryanthemums and other 
Ficoidez, natives of hot, sandy plains in South Africa. 
The succulence here acts as a reservoir for water. 
Special precautions are taken against evaporation. We 
see the first symptoms of such a habit in some English 
dry-soil saxifrages. 
Proximity to the sea, whether the plant grows in sand 
or mud, also tends to produce succulence. This effect is 
seen casually in many seaside weeds, and habitually in 
such cases as samphire, /xzla crithmoides, Spergularia 
rubra, Cakile maritima,and common scurvy-grass. Seda 
maritima is in this group the exact analogue of Sedum 
anglicum, while Salicornia is similarly the analogue of the 
leafless cactuses. Compare also Sa/sola kali. There is 
a somewhat similar tendency to fleshiness in certain 
freshwater weeds of moist spots, such as Chrysosplenium, 
and many saxifrages. 
In such a brief sketch as the present it is impossible to 
do more than allude in passing to sundry more special 
developments of leaves, for protective or other purposes. 
One development of this character is seen in the growth 
of prickly tips (Agave, Aloe, Salsola, Juncus acutus, Bro- 
melia pinguin), or of prickly edges (thistles, Car/ina, holly, 
Stratiotes, Dipsacus, Rubia peregrina). Such prickles 
may be purely defensive, or they may assist the plant in 
clambering (S¢e//ate, Smilax, hop). Again, the leaf as 
a whole may be reduced toa prickle, as in gorse, where 
the very young seedling has trefoil leaves like its allies: 
but these give way gradually to entire lanceolate blades, 
and finally to mere thornlike spines. Another very 
different development is that of the insect-eating plants, 
which grow in very boggy spots, and so require animal 
matter not yielded them by the roots. Our English sun- 
dew (Fig. 41) is an example of the first step in such a 
process ; essentially its leaves belong to the obovate tufted 
or rosetted type represented by the daisy, only a little 
exaggerated; but they have been specialised for the 
insect-eating function by the evolution of the little glan- 
dular hairs. Even simpler is the type of the butterwort, 
NATURE 
[March 29, 1883 
which belongs to the same foliar class as the London 
Pride, Draba aizoides, Samolus Valerandi, Sempervivum 
tectorum, &c., but with the edges folded over so as to 
inclose its insect prey. From these simple forms we 
progress at last to highly specialised types like Dionca 
(Fig. 42), Sarracenia, Darlingtonia, Nepenthes, and 
Cephalotus. Once more, the connate form in opposite 
leaves (Dipsacus, Chlora) or the perfoliate in alternate 
ones (Buplewrum) may be due, as has been suggested, to 
the facilities these arrangements afford for storing a little 
reservoir of water, which acts as a moat to protect the 
flowers from climbing ants. But such minor selective 
actions are too numerous and too diversified to be 
noticed in full here; it must suffice to point out the 
general principles upon which the forms of leaves usually 
depend, leaving the reader to fill in the details in every 
case from his own special observations. 
GRANT ALLEN 
FOSSIL ALG) 
Pee publication of Saporta and Marion’s “ Evolution 
of the Cryptogams” (see NATURE, vol. xxiv. p. 75, 
558) has been followed by a work in which Dr. Nathorst 
has endeavoured to prove that nearly the whole of the 
supposed fossil marine Algae, especially from the older 
rocks, are either tracks of Invertebrata or were produced 
by mechanical agency. ‘ Floridez, Laminariez, Chon- 
driteze, Alectorurideze, Arthrophycez, Bilobites, and other 
algze ; comprising among them forms curious and remark- 
able by the regularity of their branching thallus, their 
phyllome with raised periphery and striated surface; all 
had disappeared as if by enchantment, and in their place 
there remained but tracks of Invertebrata, moving upon 
the ooze, swimming or creeping, and impressing the ex- 
tremities of their tentaculary palpze around them, or of 
larvee gliding through the slimy mud.” When these are 
insufficient, the movement of water acting on inert bodies, 
or waving tufts of sea-weed, are appealed to, for no fossil 
imprint either sunk or in relief, unless preserving car- 
bonaceous matter, is admitted in Dr. Nathorst’s hypo- 
thesis to have ever been a plant. This view is ener- 
getically combated by Saporta in the present work. The 
issue however does not very materially affect either the 
general theory of plant-evolution, as traced by Saporta 
and Marion, since this relies but little upon the evidence 
of doubtful fossil algz, or the succession of marine alge in 
time, which seems to have been probably Laminariez, 
Fucacez, and Floridee. The main point in dispute is 
whether the supposed. primordial alga, Eophyton and 
Bilobites, are of vegetable or of other origin. There are 
numerous @ friovi reasons for supposing plant life to 
have existed in palzeozoic seas, and the complexity of life 
seen in even the older rocks renders their presence almost 
a necessity. The question is whether certain impressions 
which are abundant in Silurian rocks reproduce some of 
these forms, or whether we are still without indications 
of the primeeval alge. 
Dr. Nathorst appears to rely very greatly upon the fact 
that many of these supposed sea-weeds are marked in 
relief upon the under-sides of slabs, proving, as he sup- 
poses, that they are the filling-in of furrows, and also 
upon the very general disappearance of all trace of car- 
bon. In denying the plant-origin of certain impressions 
lately described as alga by Prof. Walter Keeping in the 
Geological Magazine, he lays particular stress on the 
former hypothesis. Saporta however devotes two or 
three pages to clearing up this, as he believes, mis- 
conception. The fact that very unmistakable impressions 
of even terrestrial plants do occur in this condition, is 
known to most collectors of them, and is explained by 
the author as follows :—A plant-stem of sufficient sub- 
™ «A propos des Algues Fossiles.’? Le Marquis de Saporta. (Paris: G. 
Masson, 1882.) 
