PHANEROGAMIA 



503 



sacs joined by the continuation of the filament, the CONNECTIVE 

 (Fig. 472). According to whether the thecse are turned inwards, 

 i.e. towards the whorl of carpels, or outwards, the anther is described 

 as INTRORSE or EXTRORSE. The opening of the ripe theca depends 

 as a rule (except in the Ericaceae) on the peculiar construction of 

 the hypodermal layer of the wall of the pollen-sac. This is called the 

 fibrous layer or ENDOTHECIUM. On the other hand, in the Gymno- 

 sperms (excluding Ginkgo, cf. p. 488), as in the Ferns, the dehiscence 

 is effected by means of the external layer of cells (exothecium). 

 As a rule the septum between the two pollen-sacs breaks down, so 

 that they are both opened by the one split in the wall. The 

 microspores in anemophilous plants are smooth, dry and light, and 

 adapted for distribution by the wind. In entomophilous flowers, 

 on the other hand, the exine is frequently sticky or provided with 



Km. 473. .1, I'ollt'ii-^niin of L'ucurliita 1'epo (x 240) ; 

 D, section of polleM-^niiii of Citcurbita vefruni.--n. 

 Allowing one of the lid-likf ar^as through which 

 the pollen-tubes protrude (x 540). 



KICJ. 474. Pollen-grain of Ml < xil cestris. 

 N, Spinous projections of the exine ; s, 

 vertically striated layer of the exine ; 

 p, the same seen from above ; n, places 

 of exit of ]>ollfii-tul>fs. (After A. 

 MEVER.) 



spiny projections, and the pollen-grains are thus enabled to attach 

 themselves better to the bodies of the insect visitors. They also 

 differ from the pollen-grains of the Gymnosperms in having more 

 or less numerous spots in the wall prepared beforehand for the 

 emission of the pollen-tube (Figs. 473, 474). Various modifications 

 of the androecium result from the cohesion, and branching of the 

 stamens, and will be described in the special part. Sterile stamens 

 which do not produce fertile pollen are termed STAMINODES. 



The flower is terminated above by the GYNAECEUM. The CARPELS 

 composing this may remain free and each give rise to a separate 

 fruit (APOCARPOUS GYNAECEUM) or they unite together to form the 

 ovary (SYNCARPOUS GYNAECEUM). The carpels, as in the Cycadaceae, 

 as a rule bear the ovules on their margins, on more or less evident 

 outgrowths which are termed PLACENTAS. In apocarpous gynaecea 

 the ovules are thus borne on the united margins of the carpels, 

 each margin bearing a row of ovules (Fig. 476). This is termed the 



