4, PRELIMINARY REMARKS. 
mation is negatived by every other point of structure. There can be no doubt, however, 
that fertilization in the Orchidacec is really accomplished by the agency of insects. The 
correctness of the foregoing résumé, as to the structure and relative positions of the 
sexual parts of the flower in the Asclepiadacee, is well confirmed by the admirable 
analytical figures drawn by Rio Creux, under the auspices of Prof. De Candolle, in 
thirty-seven genera of the family: here the clavuncle is constant, generally peltate and 
pentagonal, sometimes club-shaped or subcylindrical; but in all cases the five angles of 
that process are glandiferous, exuding a peculiar juice: the development during growth 
appears to me to show that self-fertilization follows as a necessary consequence, without 
the aid of external agency. 
On the other hand, we cannot pass over the different view entertained by the cele- 
brated Robert Brown, who states unequivocally that, in all the Asclepiadacee ** that 
have been hitherto examined, the absolute necessity for the assistance of insects is mani- 
fest?” *; and in proof of this he ** considered the evidence complete.” 
But neither in this memoir, nor in any other work of that celebrated botanist, as far 
as I can remember, is any evidence produced to show how this agency is employed, or 
why it is necessary ; on the contrary, the phenomena so minutely described by him, 
showing how the pollen tubes are generated, how they force their way through zigzag 
obstacles (J. c. p. 725) till they reach the stigma, whence they pass down the stigmatic 
channels of the style (* mucous tubes") until they reach the ovules, are all effected 
through self-ageney and natural development, without any external influence. 
An explanation may be offered in regard to the coma of the seeds. "This term is here 
confined to that process at the apex of the seed, formed by the strophiolar enlarge- 
ment of the testa around the micropyle, which bears a number of long erect hairs in 
several series: this peculiar process is found only in the ¿rue Echitee and in the 
Asclepiadacee. The term coma is excluded from those instances where it has been 
employed to denote the excurrent mass of hairs which emanate from the surface of 
the testa. 
Returning to our subject, I may here remark that the numerous observations made 
by me in many former years, the careful analyses obtained in Brazil and since my return, 
the plants I collected there, still preserved, those in the herbarium of the British 
Museum, including many original types, the care taken in the analyses of these, 
have afforded useful results, novel in great measure, all here interpolated with the 
recorded descriptions of botanists, and all arranged in consecutive order. I have also 
been able to identify with specimens in our herbaria the thirty species figured by 
Velloz under Tabernemontana and Echites. The evidence thus obtained, copious 
as it is, is not sufficient to form a complete monograph of all the South-American 
species. To my extreme regret, illness and other causes haye prevented my examination 
of the rich materials in the herbarium of Kew; IÍ have not even attempted to examine 
the plants of this family belonging to the Old World; so that much more is required to 
be done, to form a complete arrangement. 
* Linn. Trans. xvi. p. 732. 
