452 ' FOURTH GROUP.— SEED-PLANTS. 



Uiricularieae mentioned above (p. 446) are dorsiventral plants. The stem has a row of 

 leaves on each of its lateral faces, and on its dorsal face lateral buds of various degrees 

 of development, which are not therefore placed in the axils of leaves. Dorsiventral 

 organs are less rare in the flowering region of the plant, in the inflorescence ; they 

 occur in some Urticaceae (see p. 410), which have the flowers only on the dorsal side 

 of the axis of the inflorescence, and in the Boragineae, in which the inflorescences 

 have leaves on their lateral faces, as in Utricularia, while a flower grows above each 

 leaf on the dorsal face of the inflorescence, as in Anchiisa. 



The not infrequent absence of bracts in radial inflorescences must not be placed 

 in the same category as the above cases of extra-axillary branching; in the latter 

 there are large leaves near the extra-axillary lateral branches, in the former case, as in 

 the Cruciferae and in the capitula of many Compositae, the formation of leaves on the 

 branching axis which bears the flowers or the branches of the inflorescence is entirely 

 suppressed, and there are no leaf-axils near which the branch can arise ; yet they 

 grow as if there were leaves actually there. It will be well to refer to what was said 

 above (page 410), about this change in the conditions of the branching in passing 

 from the vegetative to the floral region of the plant, and on the frequent displacement 

 of the bracts upon their axillary shoot. 



Adventitious shoots are of rare occurrence in Dicotyledons, as in Phanerogams 

 generally ; those are well known which are formed exogenously in the indentations 

 on the margins of the leaves of BryophyUiim calycinum, and afterwards separate from 

 the plant and are capable of a further development like gemmae ; adventitious buds are 

 sometimes found according to Peterhausen^ in Begonia coriacea in the form of small 

 bulbs on the surface of the peltate leaf, where the primary veins radiate. On the 

 adventitious shoots on the leaves of Utricularia see Pringsheim 'K Adventitious 

 shoots are more common on roots, as in Linaria vulgaris, Cirsiiim arvense, 

 Populus lre?}mla, Pyriis Mains and many other plants according to Hofmeister. 

 The shoots which spring from the bark of the stems of older trees are not necessarily 

 adventitious, since the numerous dormant buds of woody plants may often be capable 

 of development after having been long hidden out of sight. 



The leaves of Dicotyledons display a greater variety of position and form than 

 those of all other classes of plants put together. The usual whorl of two cotyledonary 

 leaves in the seedling is either continued in decussate pairs, or passes into two rows 

 of alternate leaves, or into whorls of several members, or into spiral arrangements 

 with very various divergences. The more simple arrangements, especially that of 

 decussate dimerous whorls, are usually constant in entire families ; the more com- 

 plicated are usually inconstant. Axillary shoots usually begin with a pair of opposite 

 or alternate leaves, which stand right and left of the median plane of the subtending 

 leaf. 



It is simply impossible to give a sketch in a brief space of the forms even of 

 foliage-leaves, not to speak of scale-leaves or cataphyllary leaves on the underground 

 parts of stems and the scale-leaves which envelope peristent buds, or of bracts and 



' Beitr. z. Entw. d. Brutkno-pen, Hameln, 1869, where various instances are discussed of 

 axillary shoots in Dicotyledons which develope as deciduous gemmae, as in Polygonum viviparum, 

 Saxifraga gratmla(a, Dentaria bulbifera, Ranunculus Ficaria. 



^ Zur Morphol. d. Utricularien (Monatsber. d. k. Akad. d. Wiss. Berlin, Feb. 1869). 



