98 THE FLOWERING PLANT. 



stamens. Their exterior is covered with epidermis, which fre- 

 quently produces hairs, and may possess stomata. The filament 

 and connective are traversed by a central vascular bundle, and 

 the ground-tissue is made up of parenchyma. A cross-section of 

 a young buttercup bud will give numerous thin slices of anthers. 

 In these it will be seen that each lobe contains two compartments 

 filled with pollen grains, and hence called £>ollen sacs (cf. fig. 51). 

 The pollen grains have been formed by active division of paren- 

 chymatous cells belonging to the ground-tissue. Later on the 

 party-wall between the two sacs in each lobe breaks down, and 

 their contents escape to the exterior by formation of a slit. 



Pollen grains are mostly spherical. They are invested by two 

 membranes, a firm outer one, the extine, often produced into 

 spines, knobs, or ridges, and in the fir into a pair of little air 

 bladders (fig. 45, C) ; and a delicate inner one, the intine. Within 

 the coatings are two or a few small cells in gymnosperms and 

 protoplasm with two nuclei in angiosperms. 



The Carpels depart even more widely from the ordinary leaf- 

 type than the stamens. They collectively form the pistil or 

 gynoecium, which occupies the centre of the flower. 



Number and Arrangement. — In acyclic and hemicyclic flowers 

 an indefinite number of carpels are frequently present. The 

 ordinary cones of the Scotch fir consist of a large number of 

 spirally arranged woody scales (fig. 45). These are carpels, or, at 

 any rate, outgrowths from them. We have previously seen (p. 75 ) 

 that a large number of carpels are present in the hemicyclic 

 flowers of the buttercup. It is probable that two alternating 

 whorls of carpels are typical for the cyclic flower. One entire 

 whorl, however, is generally suppressed, and the remaining one 

 is very frequently reduced. Dicotyledons often possess five or 

 four carpels, as in geranium and holly respectively, but a smaller 

 number is extremely common. Thus, pansy (fig. 51) has three, 

 wallflower, dead nettle and foxglove two, and papilionaceous 

 flowers only one. A few British monocotyledons, as flowering 

 rush and frogbit, retain six carpels in alternating whorls of three 

 each. Far more commonly the inner whorl is suppressed, so 

 that three carpels only are present, as in lily (fig. 38), tulip, 

 snowdrop, and orchis. Still further reduction takes place in 

 most grasses, which have only two carpels, while duckweed, wild 

 arum, and maize retain but one. In male flowers carpels are, of 

 course, absent, and in this case they may have been suppressed 

 or else never have existed. As mentioned previously, neuter 

 flowers contain no essential organs. 



Most flowers are bisexual, i.e., possess both stamens and car- 

 pels, some are unisexual ; and are then male, with stamens only, 



