2O4 



SYMMETRY OF THE SPOROPHYTE 



those relatively few cases among Bryophytes where the radial symmetry is 

 departed from. 



Among the Liverworts the only recorded example of departure from 

 the radial symmetry is that of Monocka : here the sporogonial head, 

 which is borne upon a cylindrical seta, is curved over to one side, and 

 it dehisces along the upper surface by a longitudinal slit, the whole 

 capsule widening out later into a spoon-like form. Examination of 



developmental stages shows that 

 the young sporogonial head is 

 cylindrical in structure. The 

 accepted version of this is that 

 the capsule is developmentally 

 four-valved, but that dehiscence 

 is by one slit only, and accord- 

 ingly that the four valves remain 

 coherent ; but transverse sections 

 of the mature capsule show no 

 evidence of a structural dorsi- 

 ventrality : the transverse section 

 is radial up to maturity, and 

 there is no apparent structural 

 provision for dehiscence. The 

 natural conclusion will be that 

 JMonodea shows only a slight, 

 and ontogenetically late and 

 unimportant deviation from the 

 inu-n,,, i. Capsule open. ^=a P ophysis. usual radial type. There are no 



rri n^ii I** m Inncri Mlriina I Qi^r'tiiMi c ^ ***- t a * *- 



observations connecting this with 



II. Unopened capsule in longitudinal section. i = s 

 /,jf=leptoxylem ; .f/ = stomata on apophysis ; c/ = colu- 

 mella ; / = peristome ; ,-J.r = archesporium ; /= intercellular 

 space. III. and IV. I Marrams to illustrate the opening 

 of the capsule. (From Goebel, after Hedwi^, Vaizey, and 

 Bryhn.) 



external causes. 



The sporogonia of most 



Mosses are also of the radial 



type throughout ; but in a considerable number a more or less marked 

 dorsiventrality is seen in the mature condition. The radial construction is 

 as a rule accurately maintained in those sporogonia which grow vertically 

 upwards, such as Sf>/nt^/ntw Phascum, or Orthotrichum^ and all sides appear 

 equally developed in the longitudinal section (Fig. 103. 3). But in main 

 an unequal development is found, which is in relation to the position 

 which they assume. As maturity is approached they curve to one side, 

 and the capsule becomes oblique, or may even hang over. In these 

 cases the earlier stages of the sporogonium are radially constructed, but 

 an inequality appears on the two sides, with, it may be, a slightly 

 greater development of the assimilating tissue on one side than on the 

 other, as in Fnnaria (Fig. 103. 5): or this may be carried to such an 

 extent that the sporogonium is markedly lopsided, as it is found to be 

 in the Buxbaumieae (Fig. 103. 8, 9, 10). 



