1236 A TEXTBOOK OF THEORETICAL BOTANY 



adaxial side of the carpel, from which, at maturity, it appears to arise. The 

 appearance may be accentuated by subsequent upward swelHng of the 

 dorsum as the ovule develops. In the bicarpellary gynoecia of Labiatae and 

 Boraginaceae the two styles unite secondarily on reaching the gynobasic 

 position, only the stigmas remaining free, though in other cases {e.g., 

 Dictamnus, Nolana) the union may be no more than a close adhesion. The 

 united styles stand over the axis of the receptacle and look like a direct 

 continuation of it. The whole process is only an exaggeration of what 

 commonly occurs in the development of peltate carpels, for example in 

 Umbelliferae, where, however, the adaxial sill grows up to meet the dorsum, 

 so that the style remains at, or near, the carpel apex. 



The receptive surfaces of stigmas are generally papillose or covered with 

 short, unicellular hairs from which a mucous fluid, containing sugars, is 

 excreted when the flower is ready for pollination. 



The period of excretion may be quite short and the time during which 

 pollination is possible is correspondingly limited. Absence of excretion 

 except at the appropriate time is one of the most general safeguards against 

 self-pollination in hermaphrodite flowers. The fluid serves the double 

 purpose of retaining pollen grains and facilitating their germination. The 

 surface of the stigma is morphologically the termination of the conducting 

 tissue, with which it is continuous, and has similar characteristics. The 

 stigmas of wind-pollinated flowers have much longer hairs than those of 

 insect-pollinated flowers and these hairs are often multicellular. They 

 act mechanically as pollen-collectors and produce little or no excretion. 



Hairs of another sort are produced on the styles or stigmas of many 

 plants as a sweeping apparatus or collector of pollen from the anthers of the 

 same flower. Sweeping hairs around and below the stigma are well known 

 in many Papilionaceae, e.g., Phaseohis, Galega, Cicer. They sweep upwards 

 pollen already shed from the anthers, when the keel, which encloses the 

 style, is depressed. The stigmas of many Compositae have a mop of bristles 

 which effectively sweep the pollen out of the tube of introrse anthers, 

 through which the style grows upwards. An analogous action in some 

 Lobeliaceae is performed by a saucer-shaped outgrowth around the style, 

 below the stigmas. In many Campanulaceae the whole style is thickly 

 covered with collecting hairs, against which the anthers dehisce introrsely. 

 Both here and in some Compositae the sweeping upwards of the pollen 

 may be assisted by contraction movements of the stamen filaments. 



Pollen collection may also be assisted by expanded flaps around the 

 stigma (Fig. 1 193). The Goodeniaceae and Brunoniaceae have their stigmas 

 surrounded by a pollen chamber enclosed by two such flaps, which are in 

 part united at their edges. They are said to be due to an upgrowth of the 

 floral disc, united to the style. Before the flower bud opens they are closed 

 over the stigma, which led to the name " indusium " being applied to them, 

 a reminiscence of the Hymenophyllaceae. The well-known shrub of tropical 

 beaches, Scaevola koenigii, may be taken as an example of the operation of 

 the pollen chamber. The chamber opens and receives pollen falling from 





