546 BOTANY PART n 



and the catkinate flowers of Angiosperms, this function is performed 

 by the wind, the elongation of the axis and the absence of an invest- 

 ment of leaves around the female receptive organ are advantageous. 

 When, on the other hand, pollination is effected by insects or birds, 

 the conspicuousness given by the presence of a perianth and other 

 attractions, such as scent or sweet-tasting substances, are necessary. 

 The form of the flower, the arrangement of the sporophylls in it, and 

 the place at which nectar is secreted must be adapted to the body- 

 form or the habits of the visiting animals. It is to this that the 

 variety of form and colour exhibited in the flowers of Angiosperms 

 must be ascribed. 



The association of hermaphrodite and unisexual flowers on the same plant 

 leads in certain Angiosperms to what is known as POLYGAMY. When herma- 

 phrodite and unisexual flowers are distributed on distinct individuals we have 

 andro- or gyno-dioecism ; when on the same individual andro- or gyno-monoecism. 



The perianth usually consists of two whorls of members : these 

 may be similar in form and colour (e.g. Lilium), 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 (e.g. 

 Rosa). In every complete flower two whorls of stamens or micro- 

 sporophylls come next within the perianth, and within these again a 

 whorl of carpels or macrosporophylls. The whorls alternate 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. 

 513). 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,'cL p. 591), as in the Ferns, the dehiscence 

 is effected by means of the external layer of cells (exothecium) (p. 545). 

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

 that they are both opened by the one split in the Vail (Fig. 509 A). 

 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 

 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 (Fig. 514). Sterile stamens which do not produce 

 fertile pollen are termed STAMINODES. 



