16 M. Mohl on the Structure of Annular Vessels. 



arcticus, Black-throated Diver, was ascertained to breed in 

 North Uist. I did not however find the nest of the latter 

 bird, but mention the fact upon the authority of several of my 

 friends who did so and know the species well — among others, 

 Lieut. Macdonald of North Uist. The Guillemot, Una Troile, 

 was obsen'ed with the Razor-bill in vast numbers in the end 

 of April, while crossing the Minch in the Uist packet, and 

 that too during the whole extent, or about thirty miles. Uria 

 Grylle, Black Guillemot, was found on all the rocky coasts, 

 but was nowhere very numerous. The first young bird fully 

 fledged was shot on the 14th of August. Of the Puffin, Mor- 

 mon arcticus, I saw countless myriads in St. Kilda*, where 

 they far outnumber all the other species. Aha Tarda, the 

 Razor-bill, is also very plentiful in St. Kilda, and, with the 

 Puffin, breeds also in Haskir, but in far smaller numbers. 

 When in St. Kilda I was told by some of the fowlers, that 

 the Great Auk, Alca impennis, is still seen occasionally, but 

 that none had been procured for many years back. 



IV. — Researches on the Structure of Annular Vessels. 

 By Hugo MoHLf. 



[With a Plate.] 



Dr. Schleiden lately published some observations on the 

 spiral formations in the cells of plants in the ^ Flora' (see An- 

 nals and Mag. of Nat. Hist., vol. vi. p. 35), which interested me 

 the more by reason of my having recently directed attention to 

 the same subject (Flora, 1839, pp. 81-142), and especially as 

 the result of his researches coincided with my own in all es- 

 sential points concerning the structure of the cellular mem- 

 brane of vegetables. His opinion principally differs from 

 mine in two points, viz. the order of development of the se- 

 condary membranes and fibres in the woody fibre of Taxus 

 and allied organisms, and the formation of the annular vessels. 



I must wait for a more favourable opportunity to examine 

 the first point ; but, as regards the second, 1 will state the 

 reasons which induce me to adhere to my former opinion, 

 notwithstanding Schleiden has set forth a new theory respect- 

 ing the development of the annular vessels. 



Long ago (see ' Flora,' 1838, p. 378,) have I been opposed to 

 the hypothesis, which is devoid of all foundation, although ge- 

 nerally received even in the present day, that annular vessels 

 owe their origin to the disruption of the spiral fibres of spiral 

 vessels whose fragments become afterwards united in the form 



* Where I procured a nearly white variety of this species, 

 t From the Flora, 1839, p. G73. 



