30 MORPHOLOGY OF THE ANGIOSPERMS 



or loss of the lateral traces and veins. Among leaves of the woody 

 Ranales, which, by association in this order with so many other highly 

 primitive characters, have been looked upon as simple, there are both 

 palmate and pinnate types — palmate in Aiistrobaileya and Tetracentron, 

 pinnate in others. 



The vascular structure of the petiole varies greatly. The variations 

 are in part related to form and function of the leaf, but the basic plan is 

 dependent upon the number and arrangement of the traces and their 

 freedom or fusion as they enter the petiole (Fig. 9). Within the petiole, 

 the vascular bundles may continue undivided or divide and unite in 

 their course to the blade. Their orientation may remain as it was at 

 entrance to the petiole or may change greatly. The arrangement is in 

 part an adaptation to mechanical support, U-shaped, I-shaped, and 

 hollow-cylinder types are common. 



The distribution of the bundles may be fairly constant in a family — 

 Ericaceae, Rhamnaceae — but is more commonly characteristic of genera, 

 as in the Proteaceae and Umbelliferae, where it has aided in taxonomic 

 studies. The vascular structure of the petiole, when better known, will 

 be of much help in taxonomy and perhaps in phylogeny. 



The Ontogeny of the Plant Body 



Increase in length of the plant body is brought about in the shoot by 

 apical and intercalary meristems; in the root, by apical meristems only. 

 Intercalary meristems are parts of apical meristems separated from 

 apical meristems by areas of mature or maturing tissues. They are in- 

 ternodal in stems and basal, in part, in leaves and floral organs. In 

 angiosperms, the meristems of the plant body are more complex in 

 structure and development than those of lower vascular plants. Between 

 1920 and 1950, topographical and histological aspects of the apical 

 meristem of the shoot received much attention. Details of structure 

 and development of these meristems were studied in a large number of 

 taxa, and an apparently sound basis was obtained for generalizations 

 on the morphological value and significance of their structure. 



Ontogeny of the Shoot Apex. The apical-cell theory, which arose in the 

 earlier days of interest in the ontogeny of the plant body, assumed de- 

 velopment from a solitary apical cell or group of apical initials. This 

 theory was replaced in the 1860s by the histogen theory, which holds 

 that development is by meristems that build up individually the 

 various tissues or parts of the axis. The histogen theory dominated in- 

 terpretation of apical meristems for more than fifty years, but in the 

 early decades of the twentieth century, with increasing interest in the 

 anatomy of the shoot, it was found to have little morphological value 



