PLANT ORGANS AND ORGANISMS 



169 



inally had free lateral stipules which latter gradually fused with the 

 stem, slid across it and adjacent stipules, then fused together to 

 form a median structure on either side of the stem. 



The axillary group represent stipules which stand in the axil of 

 the leaf with the stem. Such may be free axillary structures, arising 

 as distinct processes, or connate, when the two stipules unite at their 

 margins and sheath the stem, as in many species of the Polygonacecz 

 such as Buckwheat, Rhubarb, Yellow Dock, Knot Weeds, etc. The 

 sheath formed is called an ochrea. 



Modified Stipules. In some plants such as the Locust and several 

 other trees and shrubs of the Legume family, the stipules become 

 modified for defensive purposes as spines or prickles. In the Sarsa- 

 parilla-yielding plants and other species of the genus Smilax they 

 undergo modification into tendrils which are useful in climbing. 



The Lamina. This as was previously indicated represents an ex- 

 pansion of the tissues of the petiole, but in sessile leaves is directly 

 attached to the stem and so a direct stem outgrowth. 



Mode of Development of the Lamina of Leaves. The lamina of 

 leaves develops in one of six ways. 



1. Normal' or Dorso ventral. 



2. Convergent. 



3. Centric. 



4. Bifacial. 



5. Reversed. 



6. Ob-dorsi-ventral. 



The first foui will be considered. 



A. Dorsoventral (the commonest). 



(a) Dorsoventral Umbrophytic. Flattened from ab6ve downward. 

 Plants with such leaf blades tend to grow in the shade. 



(6) Dorosoventral Mesophytic. Similar to the former, but plants 

 usually grow directly in the open and exposed to sunlight and winds. 



(c) Dorsovertral Xerophytic. Similar to former, but plants not 

 only grow exposed, but exposed to hot desert conditions or to cold 

 vigorous conditions. 



(d) Dorsoventral Hydrophytic. All transitions between typical 

 mesophytic forms to those of marshy places, to swamps and borders 

 of streams and finally with leaves wholly emersed, the last a com- 

 pletely hydrophytic type. 



