THE ANGIOSPERMAE 



I I2i 



and an active corpus, and the differentiation between vegetative and floral 

 apices is purely physiological. In Datura the petals and sepals originate, 

 like the leaves, from T,,, but the stamens come chiefly from the corpus. 

 A study by McCoy of Eraser a carolinensis (Gentianaceae) showed that its 

 floral apex was homologous with that of the vegetative shoot, with a corpus 

 and two-layered tunica, the floral organs being initiated by periclinal 

 divisions in the T2 layer. Other examples could be quoted, but enough 

 has perhaps been said to indicate that it is difficult to accept the floral apex 

 as " irreducible ", i.e., inexplicable in any terms but its own. 



m 



Fig. 1099. — Ramiucidus trilobiis. Floral ontogeny. A and B, Whole flower, younger and 

 older stages. C, Carpel primordia in surface view. D, Young carpel. E, The same in 

 section. F, Mature carpel in section. 



x = sepals, /) = petals, 5/ = stamens, c/) = carpels, 0/ = ovule, /= furrow of young loculus, 

 «</= point of attachment of carpel, w = micropyle,/» = funicle, 5;^ = stigma. {After Payer.) 



The general rule in the ontogeny of floral parts is that they follow one 

 another in a definite acropetal series (Fig. 1099), the youngest organs being 

 those nearest to the growing region. Normally the latter is at the floral 

 apex, but instances of reversal may occur, in which the apex ceases to grow 

 while an intercalary zone continues active, thus causing a basipetal (or 

 centrifugal, if the expression be preferred) order of development, which is 

 most observable among the stamens. The systematic occurrence of this 

 peculiarity in certain families is dealt with in the section on the androecium 

 in this chapter. Where only two whorls of stamens exist and they develop 

 in the centrifugal order it will follow that the outermost whorl of stamens, 

 those opposite to the sepals, will arise later and lag behind the development 



