THE LIFE AND WORK OF DARWIN 217 



In 1876 appeared "Cross and Self-Fertilisation in 

 Plants," the outcome of a great series of laborious 

 and difficult experiments on the fertilisation of 

 plants, which occupied him for eleven years. The 

 book on orchids showed how perfect are the 

 means for ensuring cross-fertilisation ; the new book 

 demonstrated how important are the results. The 

 investigations of which this book was the outcome 

 were commenced with a simple experiment made 

 for quite another purpose. Darwin raised two large 

 beds, close together, of cross-fertilised and self- 

 fertilised seedlings of Linaria vulgaris, the common 

 toad-flax, and his attention was aroused by the fact 

 that the cross-fertilised plants, when fully grown, 

 were plainly taller and more vigorous than the self- 

 fertilised ones. 



In 1877 the "Forms of Flowers" was written, 

 being the development of a short paper read before 

 the Linnean Society in 1862 on the two forms, or 

 dimorphic conditions, of Primula : one of which has 

 short stamens situated in the middle of the tube of 

 the corolla, and a long style, the stigma of which is 

 on a level with the open flower. The other form 

 has long stamens reaching to the centre of the flower, 

 while the style is short and the stigma half-way 

 down the corolla, at the same level as the stamens 

 of the other form. (See Fig. 36.) Darwin showed 

 that these flowers are barren if insects are prevented 

 from visiting them, and further, that each form is 

 almost sterile when fertilised by its own pollen, but 

 each is fertile when fertilised with the pollen of the 

 other. By referring to the figures it will be seen 



