1907] 



BARNES 6* LAND— ORIGIN OF AIR CHAMBERS 



205 





can be produced in no other way. In presenting the evidence we 

 have selected only a few examples, and show in the drawings merely 

 the walls^ since to show the cell contents would only obscure the 

 essential features. 



In the Ricciaceae we have examined only Riccia fluitans and R. 

 naians, in which the origin of the air chambers is exactly alike, though 

 the later course of development seems to be different. The origin 

 of both is certainly by internal cleavage, and it is quite evident that 

 the air chamber is wholly unrelated to the sex-organ pit. Fig. 2 

 furnishes conclusive evidence 

 on both points. An apical 

 cell ( ?) is at a; at ^ is a young 

 cleft and at - an older one^ 

 which has promptly broken 

 out to the surface;- while ? is 

 a young archegonium. The 



Fig. 2 — Riccia nutans: a, apical cell (?); 



fact that this organ is not yet j, 2, air chambers; 2 young archegonium. 



overgrown 



by the adjacent 



tissues^ while the air chamber ^ . is clearly defined, shows forcibly 

 how Leitgeb erred in homologizing the sex-organ pit and the air 



chamber. In figs, j, 4, furihoi 

 stages of development are 

 shown and also the tendency 

 of the spaces to close at 

 certain points, py both at 

 surface and internally by the 

 outgrowth of cells. 



The peculiar form of the 



Fig. ^,— Riccia natans. Older air cham- thallus and the Suddcn tume- 

 bers; i internal; at p. p partial closure of . .^^ ^^ ^^^ ^.^^^^^ -^ ^ .^^.^ 



passage by outgrowing cell. 



natans makes it very difficult 



to trace the history in detail, and we are not prepared to make any 

 statement at present as to the later course of development of the 

 air chambers. The' system appears to be a complicated one; there 

 is rapid enlargement of primar)^ spaces, probably accompanied by 

 secondary cleavage and partitioning of the chambers. 



In :\Iarchantia the splitting does not take place so early in the 



