FUNCTION.] POLLEN TUBES. 225 



The first act of fecundation in plants is, therefore, usually 

 the emission of a tube by a pollen grain ; but the impregna- 

 tion of the ovule must necessarily be a subsequent and 

 perhaps different process, in consequence, among other things, 

 of the distance which the pollen tube must travel through 

 the stigmatic tissue before it reaches the ovule ; a distance 

 computed by Morren to amount to 1150 times its own 

 diameter in Cereus grandiflorus. This botanist states that, 

 in that plant and the Vanilla impregnation does not in fact 

 occur till some weeks after contact between the pollen and 

 stigma has taken place. 



It is., however, worthy of remark, that the first act of 

 fecundation produces an immediate effect upon the floral 

 envelopes, In Orchids, a flower artificially fecundated will 

 change colour and begin to fade in twenty-four hours at the 

 latest after this has happened, although the same flower 

 would have remained in beauty many days if not impregnated. 



It would, therefore, seem that actual contact between the 

 pollen and the stigma is indispensable in all cases. Orchids 

 have, however, been thought to offer an exception ; for in 

 those plants nature has, on the one hand, provided special 

 organs, in the form of the stigmatic gland and the caudicle 

 of the pollen masses, to assist in the act of fertilisation ; and 

 on the other appears to have taken great precautions to pre- 

 vent contact, by so placing the anther that it seems iiext to 

 impossible for the pollen to touch the stigma unless artifi- 

 cially applied to it. Nevertheless, it is represented by Adolphe 

 Brongniart, in a paper read before the Academy of Sciences 

 at Paris, in July, 1831, that contact is as necessary in these 

 plants as in others, and that, in the emission of pollen tubes, 

 they do not differ from other plants. These statements have 

 been followed up by Brown, in an elaborate essay upon the 

 subject, in which the results that are arrived at by our 

 learned countryman are essentially to the same effect. On 

 the other hand, the observations of Francis Bauer, and the 

 general structure of the order, seemed at variance with the 

 probability of actual contact being necessary ; and, as Brown 

 was obliged to have recourse to the supposition that the pollen 

 of many of these plants must be actually carried by insects 



VOL. II. Q 



