392 THE BIOLOGY OF FLOWERING PLANTS 



no difference as regards fertility between the results of 

 autogamy and allogamy. He gives a list of about 150 

 plants which are said to be self-sterile, but notes that this 

 is not exhaustive. Sterility of apples and pears within a 

 race is well known, and mixed plantings are resorted to in 

 practice to secure the setting of fruit. 



Even when a plant is not strictly self-sterile, the pollen 

 from another individual may be more active than, or pre- 

 potent over, " own " pollen. The tubes may grow more 

 quickly, so that the ovules are reached first by the foreign, 

 if ** own " and foreign pollen are placed on the stigma at 

 the same time. Cases of differential growth rate in pollen 

 tubes of Rumex and Lychnis have been already mentioned 

 in another connection. 



In the cases of self-sterility which have so far been 

 analysed the cause lies in some failure in the germination 

 of the pollen grain, or in the growth of the pollen tube. 

 Jost (1907) has shown that in the laburnum, Cytisus Labur- 

 num, pollen does not germinate unless the stigma has 

 been wounded. If the flower is artificially or naturally 

 selfed the grains do not germinate. If the flower is insect- 

 pollinated, in which case cross-pollination takes place, 

 slight wounds are inflicted by the visiting bee, which enable 

 the pollen to germinate. If the stigma is artificially 

 wounded, then " own " pollen will lead to fertilisation. In 

 Corydalis cava germination of " own " pollen takes place 

 if the stigma is crushed, but the tubes soon cease to grow, 

 and no fertilisation follows. In Secale cereale, the rye, and 

 Lilium hulbiferum, germination is normal, but growth of 

 " own " pollen tubes soon stops. Darwin (1868) gives some 

 remarkable instances of self-sterility in orchids, described by 

 Fritz Miiller. In eleven species " own " pollen is not only in- 

 capable of producing fertihsation, but is killed by the stigma. 

 If pollen of Oncidium flexuosum is placed on the stigma of 

 the same flower, or on that of another flower of the same 

 plant, it becomes brown and dies in five days, while pollen 

 from a distinct plant on the same stigma is perfectly fresh. 

 In Notylia, " own " pollen is killed in two days and the 



