592 WEED FLORA OF IOWA 



In some cases, as in the Hare's-ear mustard, {Conringia orient- 

 alis) and in the upper leaves of Canada thistle {Girsmm arvense) 

 the petiole is absent .and the blade is directly attached to the stem. 

 Such leaves are designated as sessile. 



Fig. 444. Sessile leaf of Thistle. 

 (After Thomfi.) 



The stipules are small leaf-like structures which appear at the 

 place where the leaf is attached to the stem. They are very often 

 absent but are conspicuous in the cinquefoils, vetches and other 

 members of the rose and pulse families. 



VEINING. 



The blade of the leaf is traversed by a frame work of fibro- 

 vascular bundles known as veins. In the leaves of grasses, sedges, 

 and rushes, the veins run more or less parallel from the base to 

 the tip of the leaf. These leaves are the parallel-veined type. In 

 the leaves of most of our common weeds, the veins are branched so 

 as to form a network. These are the netted-veined type. 



Palmate and pinnate veimng. — ^Netted-veined leaves are palmately 

 veined when the primary ribs radiate from the base of the petiole 

 as in the great ragweed. If there is only one midrib from which 

 smaller ribs extend both ways, as in dandelion, dock, goldenrod, 

 etc., the veining is said to be pinnate (meaning featherlike) . 



LEAF ARRANGEMENT. 



Sometime.} as in dandelion and evening primrose, the stem does 

 not appear above ground or is late in appearing and the leaves at 

 the surface of the ground are called radical leaves in distinction 

 to the stem or cauline leaves. 



