BRITISH FOREST TREES 47 



may be interfered with by rain, and the dioecious species 

 (e.g. willows and poplars), bear seed on the whole less 

 frequently than trees with hermaphrodite flowers (e.g. ash, 

 elms, maples), whilst of the latter those bear seed less 

 frequently which are liable to suffer from frost, or which, 

 as is also the case with oak and beech, require a favour- 

 able preceding year's growth to enable them to form 

 flower-buds. Whereas the seed of spruce, silver fir, and 

 larch ripen in about six months after flowering, the seed 

 of pines is not mature until the autumn of the following 

 year, or about eighteen months after flowering. 



As to average yield of good seed in each seed-year, the forest trees 

 maybe classed as follows : Good seed-producers Beech, oak, spruce 

 Scots pine, birch, hornbeam, elm, alder, aspen, willow ; indifferent 

 seed-producers ash, maple, sycamore, silver fir, larch. 



But as beech has on the average good seed-years only every 

 5 8 years, and as oak, spruce, Scots pine, alder, and ash 

 have them only every 3 5 years, w>ile the other species 

 fructify in shorter intervals, they have been ranged by Gayer 

 with regard to the total average annual production of seed 

 as follows (pp. tit. p. 47) : 



Producing most Birch, aspen, willow. 



then Pines, spruce, elm, hornbeam, alder. 



then Maple and sycamore, silver fir, larch, lime, 



oak, alder, ash. 

 ,, least Beech. 



In Britain, the English elm does not produce germinable 

 seed, and the lime and chestnut only infrequently, but 

 they compensate for this by their capacity of throwing out 

 root-suckers. 



Species having small, light, winged seeds produce more 

 seed than others with heavy or with wingless seeds. It is 



