186 COLOUES OF FLOWEES AS A MEANS OF ATTEACTING ANIMALS. 



Moutan of 13-15 cm., several Mexican Cactuses (e.g. Echinocactus oxygonus and 

 Tetani) and the Gourd (GvxiV/rhita Pepo) flowers of 10-12 cm. diameter. 



Another method by which flowers are rendered conspicuous to the naked eye is 

 the massing together in bunches, spikes, racemes, umbels, and capitula. A single 

 flower of the Elder {Samhncus nigra), with a diameter of only 6-6 mm., would be 

 scarcely visible on its dark background at a distance of 10 paces, while a thousand 

 or fifteen hundred of such flowers arranged in a flat nosegay of 16-18 cm. diameter 

 show up quite plainly at the same distance from the dark-green foliage. Even 

 the flowers of the American weed, Galinsoga parviflora, recently established in 

 Europe, which are amongst the smallest in the world, having only a length of 1 mm. 

 and diameter of O'S mm., become so conspicuous when crowded together in great 

 numbers on a flat disc that they may be easily distinguished by the eye at a 

 distance of 15 paces. The flowers of about 10,000 different Composites, 1300 

 UmbeUiferEe, and innumerable Valerians, Pinks, Stitchworts, Spiraeas, Papilionace^, 

 and Labiatse are only visible at a distance because crowded together. If isolated 

 their minuteness would prevent them from being noticed. 



Very often it is only a part of the flowers which, when collected into umbels, 

 racemes, and capitula, make the whole conspicuous. In species of Iberis belonging 

 to the Cruciferse (e.g. Iberis amara, gibraltarica, umbeUata; of. fig. 252 1''), in most 

 Scabiouses (e.g. Scabiosa Oolv/mbaria, cretica, grcmiinifolia), and in not a few 

 Umbelliferge (Baucus, Heracleum, Orlaya; cf. figs. 252 ^^ and 252 "), the flowers 

 growing at the circumference of the umbel or capitulum show an enlargement on 

 one side; i.e. those petals which are turned away from the centre of the inflores- 

 cence are considerably increased and look like short rays proceeding from the 

 periphery. Some Cruciferse of the genera Alyssum, Bentaria, and Sisymbrium- 

 are also remarkable instances. It cannot be said of these that the flowers standing 

 at the circumference of the umbellate group are really enlarged on one side, yet 

 they have the same appearance as the radiating flowers. This is accounted for by 

 the fact that the petals do not fall off after the deposition of pollen on the stigmas, 

 but remain behind, fold together like the leaves of a book, and, what is still more 

 remarkable, after a little while grow together. When the flowers of Alyssum 

 montanum,, Wulfenianum, and cuneatum (of. fig. 252^) reach the highest point of 

 their development, when pollen is formed by their anthers, and honey for insects 

 stored in the flower base, the yellow petals have a length of 3-4 mm.; but when 

 once the anthers have given up their pollen and the flower base is cleared of its 

 honey, when the stigma has dried up and the ovary has already grown into a small 

 fruit, then the petals attain a length of 6-7 mm. (cf. figs. 262® and 252''). Thus, 

 while the flowers which have just reached maturity and stand in the centre of the 

 group are small and insignificant, those at the circumference display enlarged petals 

 radiating outwards, thus rendering the whole inflorescence conspicuous. In other 

 words, the older flowers are actually occupied in the allurement of insects for the 

 advantage of the younger ones. 



The difference between the peripheral and central flowers of one and the 



