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THE GEEMINATION OF POPLARS. 



By Florence H. Woolward. 



(Plate 487.) 



The genial summer of 1906 is thought to have had a beneficial 

 effect upon trees by maturing and strengthening their young 

 growths, and this may, perhaps, partly account for the unusually 

 large number of Poplars which ripened seed in 1907, although 

 more unfavourable conditions for the diffusion of pollen than the 

 w r et cold spring of this year can hardly be imagined. The ex- 

 treme rarity of Poplar-seeds in this country may be caused by the 

 dioecious character of the genus, for it frequently happens that 

 trees of one sex are abundant in a district, while specimens of the 

 opposite sex are entirely absent. In my experience pistillate 

 trees are generally less common in England than staminate ones, 

 another hindrance to the production of seeds. In natural forests 

 and uncultivated land where the trees are indigenous, the distri- 

 bution of the two forms would probably be more equal. 



There is no doubt, however, that the marvellously fine powder 

 of the pollen-grains is often carried by the wind to a great dis- 

 tance, for I am informed that the large pistillate tree of Populus 

 nigra at New Eoss, in Ireland, from the seeds of which I have 

 raised young plants, is quite an isolated specimen, and a tree at 

 Kew which ripened seeds abundantly this spring is stated to be a 

 quarter of a mile from any other Poplar. Owing to miscellaneous 

 planting, especially in botanic gardens, and the proximity of trees 

 of different species, natural hybridization might frequently result, 

 and in many cases the complete origin of seeds might be difficult 

 to ascertain. 



The first seeds which germinated under my care were from a 

 tree growing in the Queen's Cottage ground at Kew, the flowers 

 of which were developed early in April, and were sent to me under 

 the name of Populus nigra. I recognized them, however, to be 

 those of P. monilifera Ait., to the staminate form of which 

 Moench, twenty-five years earlier, gave the name of P. canadensis; 

 and an examination, later in the year, of leaves from the same tree 

 confirmed my opinion. Without going into the vexed question 

 of species, variety, or hybrid, or attempting to discuss synonymy, 

 I shall in this paper refer to the tree as P canadensis. 



In June, specimens of the fruit were sent to me, and proved 

 to contain fertile seeds, which ripened in a few days and were 

 scattered about abundantly among masses of cottony down from 

 the bursting capsules. The minuteness of the seeds of this 

 species — and, indeed, of those of all Poplars — in proportion to the 

 great size of a full-grown tree, is remarkable. The largest seed 

 measures less than one-eighth inch in length, and staminate trees 

 attain a height of 130 to 140 ft., and a girth of more than 18 £., 

 with fine spreading heads visible over the tops of all other trees 

 in the neighbourhood. Specimens of P. canadensis as large as 



Journal of Botany. — Vol. 45. [December, 1907.] 2 h 



