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 Polygonacece 
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 Dorsoventral. 
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 above downward. 
Plants with such leaf blades tend to grow in the shade. 
(b) Dorfyspventral Mesophytic. —Similar to the former, but plants 
usuallv grow directly in the open and exposed to sunlight and winds. 
( c) Dorspver tral 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. 
