470 MR. JOHN MIERS ON A NEW GENUS OF THE BURMANNIACEA. 
M. Brongniart states confidently * that he never was able to see the pollen-tubes de. 
scending the style as Amici had asserted ; he searched for them with great care, but was 
unable to trace them beyond the tissues of the stigma, where they terminated. 
Subsequently, in one of his latest communications to the Linnean Society, Mr. Brown 
further stated, in regard to the ** supposed pollen-tubes,” or what he termed “ mucous- 
tubes” (equivalent to the stigmatic channels of Mirbel), which he observed in Cytinus, 
running along the style, and reaching the cavity of the ovary, their extremities there 
mingling with the ovules, that he never found any of them applied to the apices of the 
ovules, even after their enlargement +. 
Mr. Brown’s observations, as above quoted, are somewhat modified in the same me- 
moir, in regard to the Asclepiadeæ. His invaluable researches prove that the pollen-mass 
in that family consists of a congeries of pollen-grains agglutinated together as in the 
Orchidee : he there observed the protrusion of a single tube from each pollen-grain ; but 
this did not take place before the pollen-mass touched the stigma, when, by its stimulus, 
the tubes were evolved and combined together in a loose bundle, which now passed over 
the surface of the large stigma; and at the moment the latter breaks away from the two 
styles, this bundle is seen to enter their large apertures. Consequently, the styles and 
ovaries being usually very short, it is not surprising to find this bundle of tubes passing 
through the large opening of the styles into the cell of the ovary. But though Mr. Brown 
saw them in this position, as shown in plate 34. fig. 11, he states positively that he was 
never able to trace any one of the tubes further than the commencement of the placenta 
(p. 726) ; and so rarely does this happen, that it was only in a single species of the family 
that he was able to see consecutively “ the rupture of the pollen-mass, the production and 
protrusion of the pollen-tubes, their union into a cord, with the course and entrance of 
this cord into the cavity of the style” (p.729). He describes the pollen-tube as being very 
transparent, yoy inch in diameter, neither branched nor jointed, and with no apparent 
interruptions in its cavity ; and, though sometimes of a comparatively considerable length, 
it cannot be said to be emitted from the pollen-grain, but is manifestly an elongation of 
its inner membrane (p. 725). 
In regard to the fertilization of the ovule in Orchidacee, Mr. Brown states (p. 709) 
that, “at the period when the nucleus has acquired more than half the size it attains 1 
the ripe seed, a thread may be traced from its apex very nearly to the open end of the 
testa, or to the apex of the original nucleus, consisting of a simple series of short cells, 
m one of which he observed a circulation of very minute granular matter, the lowermost 
cell being the rudiment of the future embryo ;” but there is no attempt here to show 
that there is any connexion between this thread and a pollen-tube. , 
Prof. Henfrey, in his memoir * On the Development of the Ovule in Orchis "ee 
Trans. vol. xxi. plate 2. fig. 27), portrays the growth of the confervoid filament just me 
voned; showing it in figs. 15 e£ seg., together with the earliest contact with it of whs 
Conci to be a pollen-tube, but which is manifestly identical with the re 
cous-tube” so well described by Mr. Brown, agreeing with it in all essent 
a 
Ann. Sc, Nat. xx. 332, + Linn. Trans. xix. 227. 
