4 



3 



ESSENTIALS OF VEGETABLE PHARMACOGNOSY 



len is, by the stiff hairs upon the bacus of the visiting insect, to which reference has 



the style branches, torn out from its recep- 

 tacles and exposed to such agencies of 



transportation as may be prepared to act 

 UDou it. Cases are even known in which 



the tearing out of the pollen in this way 

 is effected by a spasmodic shortening of 

 the stamens \ipon the instant of contact 



by a visiting insect, the pollen being by 



the same process at once discharged upon 



the body of the latter. After the removal 



of the pollen, or after the death of such methods of securing cross-pollination, it 



grains as fail to be removed, the style is yet true that the great majority of in- 



brauches separate in readiness to receive stances are not subject to classification 



the pollen brought from some other flower. and must be denominated special, or else 



been made, is very frequently interfered 

 with for the purpose of forcing the insect 

 into such a position as shall favor or com- 

 pel the removal of the pollen, a labor 

 which is by no means agreeable to it and 

 which it not rarely seeks to avoid, as in 

 the case of the bee, which cuts a hole at 

 the base of some corollas. 



In spite of the possibility of thus effect- 

 ing a rough classification of some of the 



This method, or some modification of it, is 

 very common among the Compositae, and 

 illustrates how the study of pollination 

 serves to ex^ilain many modifications of 

 flower-structure otherwise inexplicable, 

 and why the possession of the latter is re- 

 garded by the biologist as indicating a 

 higher stage of development. The ex- 

 planation of the case of dimorphism ex- 

 hibited in Figs. 234 and 235 is "as fol- 

 lows: — An insect visiting flower No. 1 

 and thrusting his proboscis deeply into 

 the corolla tube in search of nectar brings 

 his body iuto contact with the stamens, 

 and pollen is deposited upon it. The next 

 flower visited may be one of the same kind 

 or one with the long style. If the latter. 



then the portion of the body which is now- 

 covered with the pollen is brought mto 



contact with the stigma, npon which the 

 pollen is deposited. At the same time a dif- 

 ferent part of the body is being laden with 

 pollen from the short stamens of flower 

 No. 2, to be depot=itod upon the short pistil 

 of still another flower, similar to No. 1, 

 If i)erchance two flowers of the same 

 form are visited in succession the result 

 is that an additional deposit of pollen is 

 secured, or at most a portion of the poiien 

 already being carried is left upon the 

 stamens of the visited flower. In conclu- 

 sion it r.iay be said that even. if, by some 

 failure in the provisions here described, the 

 flower should become self-pollinated, we 

 have excellent reasons for believing that 

 poll in from a different flower which might 

 Hbe lepositedatthe same time would find an 

 .advantage accorded to it by which it 

 ■.vould be enabled to first reach and fer- 

 tilize the ovules. 



The assuming of a form convenient for 



that they combine some special arrange- 

 ments with such general methods as have 



been described. 



Flowers which are self-fertilized before 

 expansion are Cleistogamous. Occasion- 

 ally fertilization takes place without the 

 removal of the pollen from the anther. 



F 



/ 



g.^^7. 



The pollen thus transferred to the stig- 

 ma must be fixed there in order that fer- 

 tilization may follow pollination. This 

 process is effected by contrivances little 

 less elaborate, although more minute, than 

 those which have been described. These 

 contrivances relate in part to peculiarities 

 of the pollen which will be found de- 

 scribed in the part relating to histology. 

 As regards the stigma, fixation is effected 

 most generally by means of the viscid se- 

 cretion to which reference has been made. 

 Appendages in the form of hairs, scales, 

 or papillae (Fig. 237) are very common. 

 In some cases the divisions of the stigma 

 are sensitive and close elastically upon 

 the pollen as soon as it is deposited. With 

 the fixation of the pollen upon the stigma 

 pollination is completed and fertilization 

 begins. 



