INTRODUCTION. 5 



sion to refer to plants of tMs kind in the following 

 chapters, I will here gire a few details respecting them. 

 It is familiar to every one that the ray-florets of the 

 Compositse often differ remarkably from the others ; and 

 so it is with the outer flowers of many Umbelliferse, 

 some Cruciferse and a few other families. Several 

 species of Hydrangea and Viburnum offer striking 

 instances of the same fact. The Eubiaceous genus 

 MusssBnda presents a very curious appearance from 

 some of the flowers having the tip of one of the sepals 

 developed into a large petal-like expansion, coloured 

 either white or purple. The outer flowers in several 

 Acanthaceous genera are large and conspicuous but 

 sterile ; the next in order are smaller, open, moderately 

 fertile and capable of cross-fertilisation; whilst the 

 central ones are cleistogamic, being still smaller, closed 

 and highly fertile ; so that here the inflorescence con- 

 sists of three kinds of flowers.* From what we know 

 in other cases of the use of the corolla, coloured bractese, 

 &c., and from what H. Miiller has observed f on the 

 frequency of the visits of insects to the flower-heads of 

 the Umbelliferse and Compositse being largely deter- 

 mined by their conspicuousness, there can be no doubt 

 that the increased size of the corolla of the outer 

 flowers, the inner ones being in all the above cases 

 small, serves to attract insects. The result is that 

 cross-fertilisation is thus favoured. Most flowers wither 

 soon after being fertilised, but Hildebrand states J that 

 the ray-florets of the Compositae last for a long time, 

 until all those on the disc are impregnated ; and this 

 clearly shows the use of the former. The ray-florets. 



♦ J. Scott, ' Journal of Botany,' men,' pp. 108, 412. 

 London, new series, vol. i. 1872, J See his interesting memoir, 



pp. 161-164. 'UeberdieGeschleohtsTorhaltnissc 



t ' Die BefruchtuDg der Blu- bei den Compositen,' 1869, p. 92. 



