CHAP. V. FUNCTION OF REPRODUCTION. 265 



their growth. The chances which threaten the fail- 

 ure of seed in dioecious species are diminished by the 

 occasional development of a few flowers of an oppo- 

 site sex among those which otherwise characterize 

 the separate individuals ; and it is well authenticated 

 that cases occasionally occur, where willows which for 

 many years had borne flowers of one sex only, have 

 afterwards changed their character and begun to bear 

 only those of an opposite sex. 



(26l.) formation of Pollen. Before we describe the 

 action of the pollen, we shall say a few words upon its 

 formation. In this case, as in the whole account of the 

 fertilization and development of the ovule, we are es- 

 pecially indebted to the admirable researches of Adolphe 

 Brongniart, who in a memoir published in the " Annales 

 des Sciences," has combined an extensive series of 

 original observations with whatever was previously 

 known on the subject ; and placed the main facts of this 

 interesting and curious question beyond the possibility 

 of successful contradiction. To Robert Brown also in 

 this as in every department of botany, we are pre- 

 eminently indebted for important and accurate details. 

 His invaluable papers on the fecundation of Asclepia- 

 dea? and Orchideae form an important epoch in the 

 progress of general physiology. 



So soon as the anther can be distinguished in the flower- 

 bud, its cells are filled with a mass of cellular tissue,' each 

 vesicle of which contains one or more grains of pollen. 

 As the anther ripens these grains enlarge and ultimately 

 rupture the vesicles ; and the debris of the cellular tis- 

 sue then forms loose fibres intermixed with the pollen. 

 In general the grains are separate, but in some plants (as 

 the heaths) three or four grains always adhere together. 

 There is no appearance of any thing like a pedicel to the 

 separate grains, nor any scar upon them like the hilum on 

 the ovule, which' might indicate an original attachment to 

 the sides of the vesicles within which they were formed. 

 In most plants each grain is composed of two membranes; 

 the exterior presenting the various appearances de- 



