XV] 



FLOWERS OF BIRCH 



141 



flowers really a dichasium, since the median flower opens 

 first. 



Fig. 40. Flowers of the Birch. A, shoot bearing <? and ? catkins. 

 B, catkin-scale &, with bracteoles a and /3, and perianth scales 1, 2 and 3, 

 of a <? partial inflorescence seen from above, and the stamens not indi- 

 cated (cf. Fig. 34). (7, a stamen. E, a catkin-scale seen from above, bear- 

 ing in its axil two bracteoles, a and /3, which subtend the three ? flowers. 

 F, the same after dissecting off the flowers. G, the same after ripening 

 (cf. Fig. 34) ; the two bracts a and ft adhering to the scale (Ei). 



On similarly dissecting off the scale of a male catkin, 

 we find it also bears two lateral adherent bracteoles 

 (a and /3 in Fig. 40, J5) and each subtends a male flower 

 of four stamens surrounded at the base by a rudimentary 

 perianth of two to four minute scales, or, sometimes, only 

 of one scale. 



The Birch, then, shows us an example of a catkin of 

 small clusters (dichasia) of flowers little more differentiated 

 than those of the Willow, but the males at least with a 

 very simple perianth. 



Many similar cases will be found in other members of 

 the catkin -bearing trees, e.g. Alder, Hazel, Hornbeam, 

 Beech, Chestnut, Oak, &c., with gradual advance in com- 

 plexity. 



I select one more illustration of such types. 



