THEIR FORM AND FUNCTION. 221 
is dilated at the very base and semiampleæicaul. In the young 
state the petioles are shorter, deeply grooved on the inner surface 
and half surround a young bud in their axil. The bud, mean- 
while, of the main axis is completely covered by its own densely 
felted leaves, which are opposite, decussate, valvate, and clasp or 
adhere to one another by means of the tomentum. 
Helianthemum Libanotis is an upright, small, twiggy shrub. Its 
leaves are narrowly linear, obtuse, strongly revolute at the margin, 
sessile, slightly tapering to the base and again dilated there, 
sheathing or rather clasping the stem (amplexicaul), deep green 
above and finely scaly, felted beneath with a fine tomentum of 
short stellate hairs. They are also opposite, decussate, completely 
sheathing the bud in the young state, and slightly connate at the 
base, even after attaining full size. 
Thus in all the above cases the species with stipules agree in 
having narrowed petioles, and the young bud is sheltered by the 
stipules. On the other hand, in the second group which has no 
stipules the petiole itself is more or less widened or sheathing 
and serves to protect the bud. 
The reason for the presence or absence of stipules seems then 
quite obvious. When present the petiole is always very narrow, 
semiterete, and tapered to the base. In the bud state the stipules 
of two opposite leaves seem like another pair of smaller leaves set 
on at right angles to the first. They touch by their contiguous 
edges and diverge at an acute angle over the enclosed bud, filling 
the open space between the leaves. Where stipules are absent 
the leaf is often sessile, and, whether or not, its base is always 
dilated and concave on the inner face, completely enclosing the 
bud up to a certain stage of its development. ; 
Let us now see whether the conformation of other species 
supports this view. In the allied genus Cistus, the species of 
which possess no stipules, the petiole is widened as in the Heli- 
anthemums without stipules. For instance: — 
Cistus villosus, like all the undermentioned species, has no 
stipules. Its leaves are oblong-oval, rugose, villous, tapering 
into a broad subwinged petiole, which is dilated at the base and 
connate for 2-24 millim., enclosing the bud when young and the 
axis when old. 
C. corbariensis differs markedly from the above in its well- 
defined petiole. Its leaves are cordate-ovate, acute, dark green, 
and very rugose, glabrous, distinctly petiolate. Petiole semi- 
