532 ON STIPULES, THEIR FORMS AND FUNCTIONS. 
their stipules, which are short, but connate in one piece like those 
of the main stem. —— 
Potamogeton densus, Linn., is the only British species which is 
exstipulate. The leaves are more than semi-amplexicaul, and con- 
volute round the axis, one within the other alternately on the two 
sides of the stem upon which they are inserted. Owing to this 
arrangement, they appear alternate as they emerge from the bud. 
The outer leaf is developed slightly in advance of the inner; that 
is, it is slightly the longer, although both are inserted upon 
the same level. 
The internodes of the stem are always relatively short, but 
near the apex they are so short that the leaves are crowded and 
appear emerging from one another; so that not only is the 
terminal bud thus amply protected, but the axis as well for some 
distance behind the apex. 
Axillary buds are protected by one leaf, placed opposite the 
leaf of the main axis, greatly reduced in size, and more than half 
amplexicaul, so that the lamina is wrapped round the bud. This 
is followed by perfect and opposite leaves. This is the only 
British species that is exstipulate. 
The large leaf-sheath in the genus Wazas affords good protection 
to the bud; its varied form also supplies specific characters. 
GNETACEA. 
In Gnetum Gnemon, Linn., the leaves are opposite, petiolate, 
exstipulate, elliptic-oblong, leathery, glabrous, persistent; the 
petioles semiterete, flattened above, articulate above the dilated 
portion, which is persistent, connate around the axis, and pro- 
longed into a short rounded tongue in front of the petiole. 
The terminal bud in the resting stage is completely enclosed by 
the short sheathing ring and prolonged tongue, which resembles 
that seen in Clusia except that it belongs more to the stem than 
‘to the petiole. 
Axillary buds are also covered by the sheath above mentioned, 
but seldom develop into branches. , 
