MESOPHYTES 
170 
Structurally, mesophytes show few marked features, on 
account of their intermediate position between the extremes. 
Their leaf structure is usually well suited to rapid transpira- 
tion, though in tropical forests a certain amount of protec- 
tion against the great midday heat is found. The epidermis 
is generally thin, the leaves dorsiventral, the stomata nume- 
rous. The plants are freely branched and expose as much 
surface to air and light as possible. 
Climbing Plants 1 . We may divide plants according 
to their habit and mode of growth into another series of 
forms of vegetation including erect plants, creepers, climbers, 
and epiphytes. Climbers form a group founded upon a 
biological peculiarity and include plants of many different 
families. They occur in most parts of the globe, abounding 
in tropical forests, where they often grow very large and 
woody and form a characteristic feature of the vegetation. 
The climbing plant throws its weight upon an external 
support, and thus evades the necessity of forming a rigid 
stem capable of standing by itself. The great bulk of an 
erect stem consists of fibres, whose function is mechanical. 
Comparatively few of these are produced in a climbing 
plant, and its growth in length is correspondingly more 
rapid, so that it is able to reach the full daylight much 
sooner than an erect plant. In a dense tropical forest this 
is a point of great importance to the species, and even in 
temperate climates many plants owe much of their success 
to this habit. 
Darwin divides these plants into four groups. 
I. Twining Plants climb by twining round their supports. 
These must not be very thick nor very smooth, and must 
stand more or less upright, otherwise the stems are not able 
to twine. Twiners exhibit many of the characters of etiolated 
plants (p. 35) ; they have long thin stems, with nodes far 
apart, and the growing and elongating part at the tip is very 
long, and shows very marked nutation in consequence 
(p. 35). The nutating tip may describe quite a large circle, 
even several feet in diameter, thus having a better chance 
of finding a support. Plants of this kind are often sup- 
posed to have been evolved in the shade of forests or other 
1 Darwin, Climbing Plants ; Schenk, Biologie und Anatomic der 
Lianen , Jena, 1892. 
