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 



