106 Mr. H assail on the Structure of the Pollen Granule. 



wet would be attended with more serious results were it not 

 for another wise and effectual provision. The individual flow- 

 ers composing a bunch, as well as the different bunches of a 

 plant or shrub, come into blossom at successive intervals, the 

 order of their expansion depending mainly, I believe, upon 

 the position of the flower, whether it be in a more direct line 

 or not for the accession of sap, on the size of the peduncle, as 

 well as on the order of the development of the flower buds 

 themselves* : thus, if the pollen at one period be destroyed, it 

 is soon replaced by the unfolding of more blossoms and burst- 

 ing of anthers : this constitutes the second means by which 

 the efficiency of the pollen is ensured. We find a third in the 

 position of the stamens in reference to the pistil, which they 

 often surround, embrace, or overhang, so as to render the ap- 

 plication of the pollen to the stigmatic surface a matter of 

 certainty. 



Again the filaments of some anthers are furnished with 

 elastic joints, which at a certain period, that is, when the 

 pollen and stigma are in a fit condition, contract and start 

 towards the pistil with a jerk, dispersing the contents of the 

 anthers around them by the suddenness and violence of the 

 motion. I may perhaps under this head refer to two instances 

 of irritability, which I have never seen mentioned in books, 

 and which are certainly not generally known. The first oc- 

 curs in Cornus canadensis : the corolla of each flower, a num- 

 ber of which are contained in a common involucrum, consists 

 of four segments ; these for some time are folded over the 

 other floral organs and meet in a common central point, 

 where they cohere by means of some secretion. Towards 

 their tips, on what is the upper surface, while thus closed, 

 arises a long spine or hair. The stamina are also four in num- 

 ber, and arise from the top of the calyx in the intervals be- 

 tween the petals, and at about half their length are bent in- 

 wards towards the pistil at a very acute angle, being retained 

 in this position by the corolla which infolds them. The base 

 of each petal, as well as the angle of inflection of each fila- 

 ment, are furnished with an elastic joint, so that if one of the 

 spines be slightly touched, not only does each segment of the 

 corolla fly back, but the stamina, the restraint being removed 

 from off them, start up, scattering the pollen of the anthers 

 around them, some of which, it is beautiful to observe, in 

 viewing this phenomenon under a low power of the micro- 

 scope, invariably alights upon the stigma, and is there re- 

 tained by means of the secretion with which it is furnished. 



* The cause of the irregular ripening of many kinds of fruits and seeds 

 admits of a similar explanation. 



