l62 



TRACHEA. 



circular outline of the relatively very large cavity in species of Cassyta: a narrow slit- 

 like inner aperture, and a very small circular outer aperture of the canal, with a 

 broadly elliptical cavity in Elseagnus acuminata : a long and narrow slit-like inner 

 aperture, and a short slit-like outer aperture, with a circular cavity in Aleurites 

 triloba— all these cases occur on the large-pitted vessels of the wood\ 



As regards the relative diameter of the different parts of a pit it is obvious, 

 from what has been premised, that it is always larger for the pit-cavity than 

 for the outer aperture of the canal. The latter is either as large as the inner 

 aperture, or it is smaller than this, and the canal thus becomes narrower outwards to 



^0) 



Fig. 6z. — 'Pteris aquilina Rhizome ; A (142) end, about ^ of a short member of a vessel ; 

 the oblique ladder-lilce end surfaces^ and a part of the lateral wall in surface view ; 3 a 

 piece of y4 aXx, magnified 37s times ; C (375} thin longitudinal section through part of a 

 lateral wall, where two vessels touch one another ; B (375) a similar section through the 

 oblique wall /"and its margin adjoining the lateral wall. At^the pits are open. 



a varying extent, and with a form corresponding to the above description : the inner 

 aperture, when of a slit-like form, and differing from the cavity, is always narrower, 

 but often longer than the greatest diameter of the latter. Slit-like bordered pits 

 placed close to one another may thus coalesce internally, in numbers from 2-6, mto 

 a common slit, as Mohl found {I.e. Figs. 6, 10, 15) in Aleurites and Elaeagnus, and 



' See Von Mohl, Ueber den Bau der getiipfelten Gefassse, Linnaea, 1842; Verm. Schriften, 

 p. 272, Taf. XII. 



