ON ORCHIDEiE AND ASC],EPIADEJ5. 549 



of the inner surface of the Corolla of some species of 

 Ceropegia, in the wool enveloping the spines in several 

 species of Mammillaria and Melocactus, and in the Coma 

 of the seed of an Apocyneous plant from Brazil : for the 

 spiral vessels in the seeds of Collomiae, first observed by 

 Mr. Lindley, and described by hiin as external, are seated 

 between the two membranes of the testa, as I have long 

 since described those of Casuarina. They differ, however, 

 in direction ; being in Collomia transverse or perpendi- 

 cular, while in Casuarina they are longitudinal, or parallel 

 to the membi'anes. 



ASCLEPIADE^. 



With regard to this Pamily, it was remarked, both in 

 the Pamphlet which was distributed in the beginning of 

 November, 1831, and in my Paper in the 'Transactions of 

 the Linnean Society,' published in 1833, that I had p 

 never been able to find the Pollen tubes descending lower 

 than the commencement of the ovuliferous portion of the 

 Placenta. But as this was far from satisfactory, especially 

 after the further course of the analogous Tabes in Orchidese 

 had been ascertained, I determined to re-examine the sub- 

 ject. 



For this pui'pose Asclepias phytolaccoides was selected ; 

 and on the 12th of the present month I succeeded in 

 tracing the Pollen tubes in that species, not only over the 

 whole ovuUferous surface of the Placenta, but also going off 

 to the Ovula, to a definite point of each of which a single 

 Tube was found in many cases attached. 



These observations I have now so frequently repeated, 

 and always with results so exactly similar, that I have great 

 confidence in the correctness of the following statement : 



In the newly expanded flower, the Ovulum in Asdepias 

 pjiytolaccoides is nearly obovate, and is compressed in the 

 same direction as the ripe seed, but in a much less degree: 

 its umbilical cord is inserted on the axis of the inner or 

 ventral side, about one fourth from the apex, and a process 



