553 
Knowledge of Rachiopteris cylindrica , Will 
The sporangia contain numerous tetrahedral spores, about 25 /ot in 
diameter ; frequently the triradiate mark may be seen on the spore-coat. 
In the case of a young sporangium, spore-tetrads, some still enclosed in the 
mother-cell membrane, are well preserved (Text-fig. 17, b). 
Textvfig. 17. a , a section of a sporangium associated with R. cylindrica ; an ., ‘ annulus ’ 
cells, x 80. (From slide K2iw, University College, London.) b , tetrad groups of spores in 
a young sporangium associated with R, cylindrica . in ., indications of the mother-cell membrane. 
In the left-hand group, the dotted line shows the position of the fourth spore, seen at a lower focus. 
X 400. (From K 21 e, University College, London.) 
V. General Discussion. 
1. The Significance of the Occurrence of a and /3 types. 
The occurrence of the a and /3 types of stems and petioles described 
above suggests three possibilities, namely, that they represent: 
i. different regions of the same plant ; * 
ii. two distinct though closely allied species ; 
ill habitat forms of a single species. 
In favour of the first consideration it may be urged that the differences 
of structure between the two forms are not so great as those existing 
between different levels of growth in a single plant of Psilotum} or of 
Hottonia? But, although the two forms constantly occur side by side, no 
transition from one to the other has been observed amongst the numerous 
specimens examined. 
With regard to the second suggestion, the points of similarity in 
structure and behaviour of a and (3 stems and petioles are too numerous to 
warrant a multiplication of species names. 
The available evidence tends to support the third possibility, indicating 
that the occurrence of a and /3 types has a bearing upon the ‘ autecology ’ of 
Rachiopteris cylindrica. 
The reduction of the xylem strand and its concentration, as exemplified 
by the presence of only one protoxylem group ; the relatively wide cortex; 
the production of air-spaces ; and the absence of mechanical tissue—features 
which characterize both stems and petioles of the (3 type, as compared with 
1 Williamson (’ 78 ), p. 351. 
2 Prankerd, T. L. : On the Structure and Biology of the genus Hottonia. Ann. of Bot., 
vol. 25, 1911, p. 253. 
O O % 
