1864. | Botany and Vegetable Physiology. 111 
ovules. The tubes of pollen, therefore, have no direct influence upon 
the original development of the ovules, but they,act first on the enlarge- 
ment of the ovarium, and by this enlargement indirectly on the ovules. 
Dr. Hildebrand deduces from all his experiments that in the formation 
of the fruit of orchids, the pollen acts in two different ways: on the 
one hand it effects the enlargement of the ovarium, and the develop- 
ment of the imperfect ovules without the pollen tubes directly touch- 
ing the ovules ; on the other hand it impregnates the ovules, directly 
touching the embryo-sac, and determining the development of one 
germinal corpuscle into an embryo. This independent action of the 
pollen upon the ovules is probably not peculiar to orchids, although it 
has thus been noticed in that family, but the remarkable facts lately 
pointed out by Darwin in his ‘ Fertilization of Orchids,’ as well as 
those just referred to, bear singular testimony to the acumen of the 
late Robert Brown, who foresaw that a patient examination of the 
structure and action of the remarkable sexual organs of this family 
would be more likely than any other means to elucidate the difficult 
subject of generation in Phanerogamic plants. 
A remarkable confirmation of Mr. Darwin’s views of the fertiliza- 
tion of orchids by insects is afforded by a South African species (Disa 
grandiflora), described in the recently issued Linnean Journal. None 
of these South African species have hitherto been examined in relation 
to their manner of fertilization. In Disa the labellum is greatly 
reduced in size, and the posterior sepal large, forming a spur containing 
nectar. The nectary thus stands behind the stigma and pollen masses, 
in a directly opposite position to that which it occupies in other orchids. 
Nevertheless, fertilization is effected by insects, by a very slight change 
in the form of the two upper petals, and in the position of the viscid 
dises of the pollen masses, which are widely removed from each other, 
and face outwards from the labellum towards the margin of the column. 
The upper sepal and two upper petals enclose the column, so that 
insects, to reach the nectar, are compelled to approach the flower in 
front ; but as the column stands in the way of the nectary, insects must 
push their proboscis or head on either side of it, in order to reach the 
nectar. In Disa the caudicles of the pollinia do not undergo the 
movement of depression, as described by Mr. Darwin, in most British 
orchids, but the caudicles are naturally crooked. In this plant there- 
fore, notwithstanding the remarkable difference in the position of the 
nectary, every part of the flower, by the aid of very slight modifica- 
tions, has become so neatly co-ordinated to ensure fertilization through 
the agency of insects.* 
In connection with the subject of fertilizing processes, a remark- 
able arrangement has been noticed, by F. Cohn of Breslau, in thistles. 
The five anthers cohere, forming a tube. At the time of flowering 
this tube is shut in at the top, enclosing the style. About this period 
* Tt may be mentioned, in connection with the interest excited by orchidaceous 
plants of late, that M. F. G. Beer has lately published an elaborate work at Vienna, 
‘On the Morphology and Biology of the Orchidacex ;’ and some remarks by Prof. 
Asa Gray, on the Fertilization of some of the North American Orchids, will be 
found in ‘Silliman’s Journal’ for September last. 
