Symmetry 175 



not involve the whole cylinder. In the horizontal rhizome of Pteridium, 

 for example, the outer ring of bundles is essentially circular in section, 

 but the group of medullary bundles tend to be flattened dorsiventrally. 

 In Selaginella the few bundles which form the vascular system also tend 

 to be flattened in the same way. This flattening may even persist in those 

 orthotropous shoots which have become radially symmetrical externally. 



Examples of internal asymmetry in the stems of seed plants are found 

 in the horizontally growing branches of woody plants. Here the branch 

 itself is not flattened but its internal structure is excentric, the pith oc- 

 cupying a position some distance above the geometrical center of the 

 branch in gymnosperms and below it in angiosperms. The nearer the 

 branch approaches a vertical orientation, the less this excentricity is. There 

 has been much discussion of the factors responsible for this internal 

 dorsiventrality (p. 356). The problem is far from a simple one and seems 

 to be involved with the specific pattern of branching characteristic of 

 the plant. 



In Leaves. All leaves are typically dorsiventral structures, but those 

 of pteridophytes and seed plants are most characteristically so. A leaf, 

 to perform its usual functions satisfactorily, must be relatively broad and 

 thin and oriented with its major surface at right angles to incident light. 



Dorsiventrality of leaves is especially evident in their histological 

 structure. The stomata and spongy tissue tend to be confined to the lower 

 part of the leaf, with the palisade layer and a continuous epidermis on 

 the upper. Some vertically oriented leaves such as those of Iris are equi- 

 facial and show no dorsiventrality, either external or internal. Others, such 

 as those of certain rushes, may actually be tubular and essentially radial 

 in their symmetry. 



The dorsiventrality of leaves in the higher vascular plants, however, 

 is inherent in something more fundamental than the orientation of the 

 blade. The vascular supply for each leaf is a segment, or group of seg- 

 ments, of the primary vascular ring with phloem outside and xylem in- 

 side. When this passes outward into the leaf as the leaf trace and finally 

 becomes the vein system, the phloem therefore tends to be on the lower 

 surface and the xylem on the upper, a characteristic dorsiventral orienta- 

 tion from the first. Even the leaf primordia become dorsiventral very 

 early. There is evidence, however, that this is the result of induction 

 from the meristematic apex, for if a region where a primordium is to 

 form is isolated from the apex by an incision, the structure that emerges 

 may be radially symmetrical (Sussex, 1955). 



In Flowers. Floral structure provides many examples of dorsiventrality. 

 The presumably primitive types of flowers are radially symmetrical, or 

 actinomorphic (regular). In many families, however, such as the papi- 

 lionaceous legumes, the figworts, the orchids, and others, especially those 



