MODIFICATIONS OF ORGANS. 105 
outer skin or epidermis (Rubus australis); (2) sensitive curv- 
ing petioles (Clematis) ; or (3) specially-developed organs for 
the purpose of climbing, called tendrils. These tendrils are 
sometimes formed of modified branches (Grape-vine), or mid- 
ribs of leaflets (Pea), or, apparently, in some cases, of stipules 
(Melons, Cucumbers, &¢.). In many plants, branches—which 
are ramifications of the stem—are produced into spines (Haw- 
thorn and Discaria), acting as defensive organs to protect the 
plant possessing them from herbivorous animals. ‘The same 
end is probably gained by spiny stipules (False Acacia), by 
spiny leaves (Holly), and by the rigidity of stems and leaves 
which characterizes many mountain-plants. Very consider- 
able development of hairs on the epidermis, as in the Rasp- 
berry, serves probably the same end; but most commonly this 
hairiness (e.g., in Celmisia coriacea), as well as the extremely 
coriaceous nature of the leaves of many plants (Veronica 
tetragona), serves to resist the attacks of insects. This latter 
function is chiefly performed by epidermal developments of 
this nature, sometimes by glandular hairs (Petunia) or by 
general viscidity (Celmisia lindsayt), and in other cases by 
something disagreeable in the juice (Dandelion and Lettuce). 
The development of the ieaf has in some cases taken the 
extreme form of catching and digesting nitrogenous substances 
—as, for instance, the bodies of insects, seeds, &e. Viscid 
parts of plants, or those parts with glandular hairs, appear 
to be avoided by insects; ‘but it is probable that in some 
cases, when insects, seeds, or any other things containing 
easily -removable nitrogenous substances are caught upon 
glandular hairs, these latter have the power of absorbing 
such substances and utilismg them 
for the use of the plant. It is 
perhaps in this way that those re- 
markable developments haye arisen 
which occur in Sundews and Venus’s 
Fly-trap. Sundews (Drosera of various 
species) are plants growing in marshy 
or damp ground, whose leaves are 
covered above and on their margins 
with glandular hairs —hence their 
name. When a sinall insect alights 
on the leaf—and the leaves are 
brightly coloured, apparently so as 
to attract iInsects—it at once becomes 
clogged by the viscid material on the 
hairs, and its struggles to escape only 
Serve to fasten it more securely, and 
in a short time it perishes. But in an hour or two aiter its 
capture the hairs in its neighbourhood commence to bend their 
Fig. 209. Leaf of Drosera 
spathulata (mag.). 
