342 
PHYSIOLOGY. 
BOOK II. 
I have remarked that, in Morrenia odorata, an Asclepia- 
daceons plant, the emission of tubes takes place to such an 
extent as to give the head of the stigma altogether the ap- 
pearance of a mass of tow. (See Botanical Register^ 1838, 
Misc. No. 129.) 
The first act of fecundation in plants is, therefore, the 
emission of a tube by a pollen grain ; but the impregnation 
of the ovule must necessarily be a subsequent process, in 
consequence 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 Orchidaceae, 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 some days if not impregnated. 
It would, therefore, seem that actual contact between the 
pollen and the stigma is indispensable in all cases. Orchi- 
daceous plants have, however, been thought to offer an excep- 
tion ; for in them 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 fertilis- 
ation ; and on the other appears to have taken great pre- 
cautions to prevent contact, by so placing the anther that it 
seems next to impossible for the pollen to touch the stigma 
unless artificially applied to it. Nevertheless, it is repre- 
sented 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 
