84 INTRODUCTION 



others. The ray-florets are usually ligulate, by which the end in view is attained 

 even more successfully. The enlargement of the marginal flowers of an inflorescence 

 also occurs in the corymbs of some Cruciferae (Iberis), and in many Umbelliferae 

 (Daucus, Heracleum, Anthriscus, Conium, Orlaya, and others). 



In many cases there is a division of labour between the flower of an in- 

 florescence : the inner ones are sexual, and devoted to reproduction, the outer 

 are asexual, and have undergone a great development of the parts that serve for 

 attraction, at the expense of stamens and carpels. Instances are afforded by 

 Centaurea, Viburnum Opulus, and others. It sometimes happens that the upper 

 flowers of an inflorescence serve to attract, while the lower ones are concerned 

 with reproduction. In Muscari comosum, for example, the compressed ear-like 

 inflorescence is only a few centimetres in length at the time of budding. Later 

 on, by gradual extension of the axis, it develops into a raceme of 20-30 cm. long, of 

 which the uppermost twenty to thirty flowers remain infertile, but assume a deep blue 

 colour, and are borne upon similarly coloured upwardly directed stalks of 1-2 cm. 

 in length. The (sessile) buds are also blue, but the open flowers with stamens and 

 carpels, about thirty to forty-nine in number, are coloured brown, and arranged in a 

 very loose and inconspicuous raceme (Knuth, ' Blutenbiol. Beob. auf d. Insel Capri,' 

 Bot. Jaarb., v, 1893, pp. 25-7). 



Many inflorescences develop flowers on one side only. This is most strikingly 

 seen in racemose inflorescences, e. g. Digitalis purpurea, Teucrium Scorodonia. If 

 the flowers were arranged regularly around the stem, they would be much less 

 conspicuous than they are when arranged unilaterally, although with this latter 

 arrangement they are only visible to insects coming from one side. Such in- 

 florescences, however, possess the further advantage that their insect-visitors, chiefly 

 bees, ascend them with the greatest regularity, as if on the steps of a ladder, 

 without passing by a single flower, as is very often the case in radially symmetrica] 

 inflorescences, with the result that some flowers often remain unfertilized. 



J. Urban (Ber. D. bot. Ges., Berlin, iii, 1885, pp. 411 et seq.) calls attention to 

 the fact that one-sidedness in inflorescences is brought about by various causes, 

 namely : 



I. By curvatures of the flower-stalks. This is the case, e. g. in Digitalis 

 purpurea L. The bracts and flower-stalks are here regularly arranged around 

 the floral axis ; but while the former retain their original position, the latter bend 

 to one side in such a way that the outermost flowers diverge 80-120'^, making 

 the inflorescence unilateral. Owing to this arrangement the flowers are adapted 

 for cross-pollination by insects with the least possible loss of time, and visits are 

 secured. They present a striking appearance on one side only, though on this 

 side they are highly conspicuous. This disadvantage is compensated by the fact 

 that the lateral inflorescences developing below the terminal one always turn the 

 side devoid of flowers towards the main axis. Even neighbouring plants are 

 similarly related to one another, for the outer inflorescences turn their flowers 

 outwards, quite independently of the strength or feebleness of the illumination. 



In Scutellaria peregrina L. and other species of the same genus, the markedly 

 unilateral arrangement of the flowers is chiefly caused by curvature of the flower- 

 stalks, combined with bending of the leaf-stalks. 



