F DICOTYLEDONS 201 
proach the Conifers, where a similar method of second- 
ary thickening seems to have been developed quite 
independently. 
While the leaves of the Monocotyledons are usually 
linear, with parallel venation, those of the Dicotyledons 
exhibit great variety in outline and venation. They 
may occasionally have simple leaves much like those 
of the typical Monocotyledons, but much oftener the 
leaves are broadly expanded, with a clearly defined 
petiole or stalk, and a broad lamina with reticulate 
venation. The base of the petiole is often provided 
with small leaf-like appendages, stipules. The ar- 
rangement of the veins varies with the shape of the 
lamina, but is always more or less clearly reticulated. 
There may be only one main vein or midrib, or there 
may be, several ‘large veins of nearly equal size radiat- 
ing from the junction of the lamina and petiole. While 
the margin of the leaf may be smooth as in most Mono- 
cotyledons, it is oftener variously indented or lobed, 
and this may be carried so far as to result in a complete 
division of the lamina into separate leaflets, and thus 
compound leaves like those of the ferns arise. The 
size of the leaves is largely dependent upon the condi- 
tions of growth, and in plants of very dry regions, or 
in some parasites and saprophytes, the leaves may be 
entirely wanting. Where leaves are entirely absent 
their place may be taken by portions of the stem, whose 
outer cells develop chlorophyll. 
The variation in the stem is quite as marked as that 
of the leaves. The stems may be herbaceous or woody; 
extremely short, even bulbous, like many Monocotyle- 
dons; or they may be enormously lengthened, slender 
