DICOTYLEDONES 



409 



often composed largely of cork. The cork may form a uniform, 

 smooth layer, as in the smooth twigs and stems of many shrubs and 

 trees, or it may be developed in irregular masses, which become 

 split by deep, longitudinal fissures and ridges, often scaling off in 

 large flakes, or shreds, as in the Plane, Hickory, or Eucalyptus. 

 These masses of cork are sometimes very thick, and may form wing- 

 like growths (Ulmus alata). Commercial cork is obtained principally 

 from the bark of Quercus suber. 



The stem in Dicotyledons is usually extensively branched, and 

 shows great variety of form, all of the principal modifications, such 

 as bulbs, stolons, climbing stems, etc., being represented. The 

 branching is almost always axillary. 



en 



The Leaf 



The leaves of Dicotyledons' show much greater variety than is 

 found among Monocotyledons. While they may be reduced to 

 scales, or be entirely absent in some parasites and saprophytes, they 

 are usually well developed and have all the parts present. The 

 typical form is a broad, stalked leaf, with reticulate venation. A 

 midrib is usually present, or if the leaf is palmately lobed, there is 

 a corresponding number of stout, radiating veins from which are 

 given off the smaller lateral veins, and these are joined by an intri- 

 cate system of smaller ones. Stipules are often present. The mar- 

 gin of the leaf is often variously cut and lobed, this finally resulting 

 in the various types of compound leaves. 

 These may be either palmately com- 

 pound (Clover, Horsechestnut, Ampe- 

 lopsis), or they may be pinnately 

 compound (Rose, Locust). Where the 

 primary segments are themselves com- 

 pound, decompound leaves result (Aqui- 

 legia, Acacia). 



Scale-leaves. Scale-leaves of two 

 kinds may be recognized: first, the 

 rudimentary scale-leaves found in vari- 

 ous colorless parasites and saprophytes ; 

 and second, the protective scales, such 

 as the thick scales of the winter-buds 

 of deciduous trees. 



Phyllotaxy. The arrangement of the 

 leaves upon the stem also shows great 

 variety. The leaves may be either oppo- 

 site or alternate, and in the latter case 

 may show a divergence of one-third, 



pb- 



r-cal 



FIG. 388. Amaranlus ddoro- 

 stachys. Longitudinal section 

 of a root, showing the origin of 

 a rootlet, from the pericycle, p ; 

 en, endodermis of the primary 

 root ; cal, calyptrogen ; pb, peri- 

 blem ; pi, plerome of rootlet. 

 (After VAN TIEGHEM.) 



