8EIJT. II 



PHANEROGAM I A 



483 



we have andro- or gyno-dicecisni ; when on the same individual andro- or gyno- 

 moncecism. 



The perianth (Figs. 425, 426) consists of two whorls of members : 

 these may be similar in form and colour, when the name perigone is 

 given to them, or may be differentiated into an outer green CALYX 

 and an inner whorl of coloured leaves, the 

 COROLLA. In every complete flower two whorls 

 of stamens or microsporophylls come next within 

 the perianth, and within these again a Avhorl f 

 of carpels or macrosporophylls. The whorls alter- 

 nate regularly with one another. The stamens 

 collectively form the androecium, the carpels 

 the gynaeceum (^). 



Each stamen consists of a cylindrical stalk 

 or filament and of the anther; the latter 

 is formed of two thecae or pairs of pollen-sacs 

 joined by the continuation of the filament, the 

 CONNECTIVE (Fig. 427). According to whether 

 the thecae 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. 530), as in the Ferns, the dehiscence 



Fig. 427. — A and B, Anterior 

 and posterior view of a sta- 

 men of Hyoscyamus niger ; 

 /, the filament ; ji, anther ; 

 c, connective (magnilied). 



Fio. 428—^, Pollen-grain of: Cucurliita Pcpo (x 240) ; 

 B, section of pollen-grain of Cucurbita verrucosa, 

 showing one of the lid-like areas through which 

 the pollen-tubes protruile ( x 540). 



FiaA'20.—Fo\len-gTa.\nofMrdvasilvestris. 

 S, Spinous projections of the exine; s, 

 vertically striated layer of the exine ; 

 p, the same seen from above ; o, places 

 of exit of pollen-tubes. (After A. 

 Meyer.) 



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

 As a rule the septum betAveen 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 



