JULY 125 



that these seeds all went to waste. In the course of 

 many years I have only found one self-sown seedling 

 of any of these species in our borders, viz. Olearia 

 numnuclari folia. Mr. Irwin Lynch, who so ably 

 filled the office of Curator of the Cambridge Botanic 

 Garden, reminds me that plants of the Composite 

 order are almost invariably self-sterile that is, to pro- 

 duce fertile seed they require cross- fertilisation between 

 individuals. Olearia and the shrubby species of 

 Senecio being usually propagated in this country by 

 cuttings, it follows that the plants produced by that 

 method are merely parts ot a single individual, and 

 being self-sterile, the seeds they bear are self-sterile also. 

 But it is impossible to believe, in view of the immense 

 number of living plants, some of them very bulky, like 

 tree-ferns, imported into Britain from Australasia, plenty 

 of fertile seeds of Composites have not found their way 

 hither, just as the seeds of British groundsel, dandelions 

 and thistles have been carried with agricultural seeds to 

 the southern hemisphere. The difference is that our 

 weeds establish themselves freely in their new environ- 

 ment, whereas Australasian weeds of the same natural 

 order make no headway in the northern hemisphere. 



The restrictive agency preventing the dispersal of 

 southern plants in the northern temperate zone may 

 be recognised, possibly, in extensive plantations of blue 

 gum (Eucalyptus globulus) and jarrah (E. rostrata) 

 in southern Spain, where the climate is not unlike the 

 average of Australia. From the former species we 

 obtain pitwood for our mines in ten years, and sleepers 

 for our railways in twenty years. Formerly we used 



