30 | ELEMENTARY BOTANY 
(Lathyrus aphaca), they perform the function of the blade ot 
the leaf, which is undeveloped (Plate I., Fig. 33). 
In woodruff, the stipules are also blade-like and similar to 
the true leaves, which bear their branches in their axils and 
are thus distinguished from the stipules. The function of 
stipules varies with their form; when blade-like and green, 
they often perform the work of foliage leaves and help feed 
the plant.. In some cases they protect young pues and drop 
off when the leaves unfold. | 
‘The leaf-stalk, or petiole, is not always present. If absent, 
the leaf is described as sessile; if present, as petiolate. The 
petiole is often modified ; thus in Lathyrus aphaca (Fig. 33) it 
is developed into a tendril by means of which the plant climbs. 
Clematis also climbs by means of its petioles, which intertwine 
with other leaves of adjoining branches of clematis, or with 
blackberry stems, covering our hedges in autumn with a dense 
growth. Nasturtium (Zropeolum) also climbs by means of its 
petioles. 
The leaf-substance of the blade, or lamina, is traversed by 
veins. If the leaf of a grass or hyacinth be compared with 
that of a violet, it will be noticed that the veins of the leat 
are very differently arranged. In the hyacinth they are 
parallel to each other, an arrangement characteristic of Mono- 
cotyledons; in the violet they branch out from the central 
vein, called the midrib, forming a network throughout the 
leaf-substance. This arrangement 1s characteristic of Dicoty- 
ledons ; so that Monocotyledons are said to be parallel-veined, 
Dicotyledons net-veined. | 
If now a sycamore leat be compared with that of lilac, it 
will be seen that they are both net-veined. In the sycamore, 
however, the main veins spring from the same point, spreading 
out like the fingers of a hand; the venation is, therefore, 
palmate (Lat. palma, a palm). In the lilac, on the other 
hand, the veins branch out from the midrib, which runs the 
whole length of the leaf; this venation is pinnate (Lat. punna, 
a feather). | | 
The blade of a leaf varies very much in shape. It may be 
