GEITONOGAMY. 321 



The capitula of the Colt's-foot (Tussilago) and of the Marigold (Calendula) 

 contain two kinds of florets. In the centre are pseudo-hermaphrodite male flowers, 

 whilst true pistillate ray-florets form the fringe of the capitulum. The latter open 

 earlier than the disc-florets, and therefore at first can only be fertilized with pollen 

 from other capitula which are further advanced. But soon the pollen is pushed out 

 of the disc-florets of the same capitulum, and is deposited in a small clump at the 

 top of the anther-tube. This pollen is conducted to the stigmas of the neighbouring 

 ray-florets by diflerent methods in the two genera mentioned. In the Colt's-foot 

 the numerous ray-florets at the periphery are expanded horizontally during the 

 daytime, but towards evening they fold up, and in this way, as they bend over the 

 tubular florets, contact with their clumps of pollen is unavoidable. The pollen is 

 transferred to the ray-florets, and when the capitula open again next morning, and 

 the ray-florets bend outwards, the adherent pollen is freed, and slips down to the 

 ripe stigmas at the base of the corolla. The process is far simpler in the Marigold. 

 The stylar branches of the ray-florets are bent inwards over the adjoining disc- 

 florets while the latter are still closed. When they open, and the pollen is swept 

 out of their anther-tubes, it of course passes inevitably to the stigmas of the neigh- 

 bouring ray-florets which are situated just above. 



The Golden-rod (Solidago), Aster (Aster), and many other Composites classed 

 together in the group of the Asteroideae, closely resemble the Colt's-foot and Mari- 

 gold in outward appearance, but their sexes are diflerently arranged. The tubular 

 disc-florets are all truly hermaphrodite, and the outer ray-florets are truly pistillate. 

 The latter mature first, and are adapted to hybridization, as we have already 

 remarked. Two days later the hermaphrodite flowers of the disc open — those 

 towards the circumference being the flrst. Their pollen is pushed out, and mean- 

 while the flowers bend slightly outwards, so that the pollen lying on the anther- 

 tubes in the form of small clumps either comes into direct contact with the ripe 

 stigmas of the marginal ray-florets or falls on to them from a short distance. 



In very many Composites the capitulum contains only hermaphrodite flowers 



with tubular corollas. The development of the flowers again proceeds from the 



circumference towards the centre of the capitulum, and in each flower, soon after 



the corolla has opened, the pollen is swept and pushed out of the anther-tube by 



the sweeping hairs or warts on the outer side of the style. The pollen forms a 



small clump at the mouth of the anther-tube, but does not retain this position long. 



The two stylar branches which have hitherto been folded together (their outer 



surface being coated with pollen) soon separate and often bend back in a curve so 



as to expose their ripe stigmatic surfaces. The pollen is thus for the most part 



detached in small crumbling balls which simply tumble down. In this way they 



reach the ripe stigmatic tissue of the older neighbouring flowers and geitonogamy 



ensues. Various contrivances are met with in these Composites to prevent the pollen 



which falls from the younger flowers from missing its mark, and to ensure its 



arrival on the stigmas of the nearest older flowers. In Homogyne alpina (an alpine 



plant related to the Butterbur) the tubular florets on the flat receptacle of the 

 Vol. II. 71 



