J. 'r. Wadswortii 161 



near St Bees, Cumberland, early in September of the same year. These 

 results give a more accurate idea of the number of seeds present in well- 

 developed flower-heads of this weed. 



From twenty-seven flower-heads 2045 seeds were obtained ; averaging 

 75-7 seeds per head ; one flower-head contained 103 seeds, whilst the 

 smallest number found in one head was sixty-two. A collection of 

 flower-heads was also made at Port Erin, Isle of Man, early in September, 

 and one head yielded the very high number of 109 seeds. 



These figures indicate the large number of seeds that may be ripened 

 in a fully-developed flower-head of this plant, and it may be of some 

 interest to make an estimate of the number of seeds that a single plant 

 of this species is capable of producing. 



Several plants of the knapweed were grown for experimental pur- 

 poses last summer, in my garden at Northenden, in Cheshire, and one 

 of them gave rise to several particularly lusty shoots ; one of these 

 bore fifty-one flower-heads, and if it is assumed that each head ripened 

 only thirty-one seeds (the average number obtained from heads gathered 

 at Prestatyn) we obtain the number 1581 seeds producible on one single 

 shoot. A vigorous plant growing in good soil would give rise to at 

 least three or four shoots, so that several thousands of seeds may be 

 produced on one plant. Long (1910, p. 175) remarks that " knapweed 

 {Centaurea nigra L.) known under a variety of names as Hardheads, 

 Hardback, Blackhead, is a too common weed of pastures and meadows," 

 and in view of the above estimate of its seed productivity, this is not 

 surprising. Were it not for the fact that several insect larvae and 

 birds feed on its seeds it would be even still more abundant. 



A germination experiment was also made to test the difference, 

 if any, between the seeds ripened in galled heads and those in ungalled 

 heads, with regard to their vitality or germinative power. 



A larore number of seeds from the ungalled flower-heads collected 

 in October were taken and mixed together ; after rejecting all seeds 

 that sho;ved evidence of injury, i.e. indications of having been partially 

 eaten by other insect larvae, 100 seeds were separated without exercising 

 any selection. These were placed on moist blotting paper in a seed- 

 germinating apparatus, and kept at a temperature of 26° C, From these 

 seeds eighty-nine seedlings were obtained, equivalent to 89 per cent. 

 In a similar manner 200 seeds were selected from the galled flower- 

 heads that were collected on the same date and these were placed in 

 the germinating apparatus at the same time. That is to say, both lots 

 of seed were collected at the same time and place, and germinated under 



