410 



FOURTH GROUP. SEED-PLANTS. 



besides cymose inflorescences, such as occur in Urtica urens and U. pilulifera^ have 

 dorsiventral inflorescences of a very remarkable form. In Urtica dioica two rows of 

 branches are formed on the dorsal side of the inflorescence, which produce cymose 

 clusters of flowers on their dorsal face only. The flat flower-heads viDorstenia owe their 

 origin to peculiar conditions of growth in the ventral side of the flowering axis, which 

 expands into a 1 flat disk and branches dichotomously. In this case also the flowers 

 grow only from one side of the head. The Papilionaceae also often have dorsiventral 

 inflorescences, the flowers being placed always on the ventral side of the axis which is 

 turned away from the primary axis, and this side is different in appearance from the 

 dorsal side before the formation of the flowers, as in Vicia y Orobus, Ononis and other 

 genera. In inflorescences of this kind with a large number of flowers, Vicia Cracca for 



FIG. 338. Young inflorescence of Isatis taurica 

 seen from above ; Jthe apex of the inflorescence, beneath 

 which the flower-buds arise in whorls of four members 

 each ; no bract is formed. The youngest buds are still 

 quite leafless, the oldest show the beginnings of the four 

 sepals. 



FIG. 339. Longitudinal section of the apical 

 region of a shoot of Clematis apiifolia ; j apex of 

 the stem, bb leaves,^- vascular bundle ; small axil- 

 lary shoots are seen in the axils of the youngest 

 leaves, and are visible in the lower pair of leaves 

 as hemispherical protuberances in their axiis. 



example, the flowers are seen to be arranged in oblique lines. For information with 

 respect to other forms of inflorescence of this kind the student is referred to the treatise 

 cited in note on page 406 \ 



On the change in the mode of branching in passing from the vegetative to the floral 

 region of the shoot Warming 2 has made some important statements, from which it 

 appears that many cases of apparently extraaxillary branching in inflorescences can be 

 derived from axillary branching as the typical mode. He says that the axillary shoot 

 and its subtending leaf must be taken together as a whole, in which the leaf which is 

 one part, and the shoot which is the other, may be developed simultaneously or one before 

 the other, and one more or less fully than the other. He then shows, that the sub- 

 tending leaf is always formed first in the vegetative region, and at first at leas* grows 

 more vigorously and rapidly than the shoot which belongs to it, and which does not make 

 its appearance till a certain number of younger leaves have been already formed above 

 the leaf in question (Fig. 339). In some inflorescences the formation of leaves antici- 

 pates that of the axillary shoots by a much less interval, as in the spikes and racemes 



1 [See also Celakovsky, Ueber ideale oder congenitale Vorgange d. Phytomorphologie (Flora, 

 1884).] 



2 Recherches sur la ramification des Phanerogames, Kopenhagen, 1872. 



