212 
MORPHOLOGY OF MEMBERS. 
former being called the leaf- stalk or petiole, the latter the blade or lamina. Or the 
lower portion of the leaf has the form of a sheath enclosing the stem and 
younger leaves like a hollow cylinder. If the upper part is flatly expanded the leaf 
then consists of a sheath or vagina and a blade ; it sometimes also happens that a 
stalk intervenes between the sheath-like basal portion and the lamina, as in Palms and 
some Aroideae and Umbelliferae. Segmentation into sheath, petiole, and blade may 
be distinguished as longitudinal, from lateral segmentation, which consists of actual 
branching, as in pinnate, deeply lobed, or compound leaves, or of a rudimentary 
branching, as in indented, toothed, and sinuate leaves. Leaves are termed compound 
in which the individual lateral pieces of the lamina are completely separated at their 
base ; while those forms are termed lobed in which the lateral branches are only more 
or less projecting portions which unite at their base. If the individual branches of a 
branched leaf are sharply separated, each branch forms independently, so to speak, a 
leaf, and is hence distinguished as a leaflet. The pinnation, like the formation of 
lobes, may be repeated. If the branches are obviously arranged in two rows the 
leaf is said to be pinnate if it is a compound leaf ; pinnately -lobed, pinnatisect, or 
pinnatifid if the divisions are incomplete ; dentate, serrate, or crenate if the lateral 
projections are very small relatively to the lamina. If, on the contrary, the branches 
or lobes of the lamina are aggregated at the end of the petiole, and radiate 
from it, the leaf is said to be digitate, palmately lobed, &c. It is termed peltate 
when the lamina is attached not by a portion of its margin, but at a point 
on its under surface (as in TropcBolum, Nelumhium, &c.). These are only a few of 
the more important forms ; the student will find in every text-book a number of 
other distinctions and terms employed in the special description of plants ^ 
As occasional appendages, which indicate a still further segmentation of leaves, 
must be mentioned stipules, ligular structures, and hood-like outgrowths. 
Stipules may be considered as lateral branches of leaves which arise at their 
very point of insertion ; they stand in pairs right and left of the base of the leaf, 
either entirely distinct from it [free) or united to it in growth {adnate); each single 
stipule is usually bilaterally unsymmetrical, and its shape is the reflected image 
of the other. Stipules are not formed until after the origin of the leaf, but then 
grow much more rapidly, and attain their final development at an earlier period; 
hence they play an important part in the position of the parts in the bud. In 
vernation they either extend by their inner margins (those facing the median plane of 
the leaf) over the back of the leaf and cover it outside either partially or entirely, 
or they extend in front of the leaf (on the side facing the stem) right and 
left, and thus cover the parts of the bud next youngest in age. In one or the 
other of these modes chambers are not unfrequently formed by the stipules, in which 
the formation of the leaves is completed, and from which they expand and unfold; 
the stipules then either also remain and unfold, or die and drop off. 
The term ligule is applied to a membranous outgrowth on the inner side of 
the leaf of Grasses at the point where the flat lamina bends out at an angle from 
the sheath; it stands transversely to the median plane of the leaf. Similar out- 
growths are also found elsewhere, as on the petals of Lychnis and Narcissus (where 
1 [Gray's Botanical Text-book, Part I, Structural Botany, 1879, gives perhaps the best English 
exposition of the current terminology of Flowering plants.] 
