XIl] 



THE GRADATE SORUS 



213 



Upon this the first sporangia appear at or near to the distal end : they are 

 followed by a basipetal succession of them (Fig. 205). In later stages distal 

 sporangia may be found already mature, while towards the base successively 

 younger sporangia may be seen. This type of sorus is characteristic of the 

 Gradatae ; it is seen in the Dicksonieae and Loxsomaceae, and it finds its 

 climax in the Hymenophyllaceae. It is found also in the Cyatheae, and 

 Onocleinae. The sporangia of all of these have an oblique annulus. The 

 spore-members from each sporangium are large in some of the Gradatae 

 (Hymenophyllaceae), but they show great fluctuations. The Ferns with 

 gradate sori are all of a middle position as regards their general characters. 



I* 



91- 



v.. 



-ti,,, 



Fig. 205. Thyrsopteris elegans, Kze. .-:/ = longitudinal section through the young 

 sorus, showing the two-lipped indusium, i, i, and sporangia, s, s, seated on 

 the receptacle, the oldest being at the distal limit of it. C= two young spo- 

 rangia. j5 = one rather more advanced. D = & sporangium with tapetum and 

 sporogenous group shaded. E, 7^= mature sporangia. A—Dy.2oo. E, F 

 X50. 



and their fossil correlatives date back to the Mesozoic Period. They do 

 not, however, represent the main body of recent Ferns, though they include 

 the largest of them. The gradate sorus may accordingly be held as deriva- 

 tive from the simple type, and it brings physiological advantages which lead 

 to a high spore-output in the largest of the Ferns that possess it. 



The difficulty of dissemination of the spores is, however, increased by 

 the close aggregation of the sporangia upon the prolonged receptacle. That 

 difficulty is met by the oblique annulus. It has been seen that in Gleichenia, 

 where there is a median dehiscence of the sporangium, elbow-room is 

 required for the mechanical ejection of the spores, and that this is only 

 possible where the sporangia are loosely arranged. Loxsonia is the only 



