740 



A TEXTBOOK OF THEORETICAL BOTANY 



is a Gymnosperm and therefore belongs to the class of Flowering Plants, but 

 it is advisable to use descriptive terms for it which have as few theoretical 

 implications as possible. 



The structure in question is axillary to a leaf and is therefore by morpho- 

 logical axiom a branch or axis, not a simple sporophyll. Its anatomical 

 structure agrees with this, so if we call it the ovuliferous axis we are stating 



:i 



Micropyle r^ 

 Pollen chamber < *' 

 Nucellus 

 Embryo sac 

 Integument 



Collar ^..^ J ■ 



^' 4 I I 



Fig. 743. — Ginkgo biloba. Vertical section through the 

 apex of an o\uliferous axis showing the paired 

 ovules each surrounded by a collar. 



nothing beyond the facts and keeping it in line with the treatment of the 

 Pine cone. 



The foliage leaf and the male sporophyll have each two trace-bundles 

 in their stalks. The ovuliferous axis has four (Fig. 744), which at once 

 suggests that it is a double structure and that the two ovules represent two 

 reduced sporophylls, the collar being perhaps a vestige of the individual 

 sporophylls. Abnormalities are not uncommon in which several ovules 

 appear on one common peduncle. Both the bi-ovulate and the multi-ovulate 

 axes may also develop stalks to the individual ovules. Where this is so, 

 each separate stalk contains two traces, like the leaf, and the common peduncle 

 has a corresponding number, which unite themselves into a vascular zone 

 like that of the stem. To complete the picture we have another class of 

 abnormalities in which the ovular stalks become definitely leaf-like. In 

 such cases there is usually a stalk bearing a broad lamina with one or more 

 ovules on its margin, the lamina being continuous with the collar of the ovule. 

 Thus it seems clear that we are dealing with a reduced strobilus, which 

 originally bore a number of megasporophylls of foliar nature, each with one 



