THE SPORE-PRODUCING ORGANS 



[CH. 



In the simplest uniseriate sori the central space is devoid of sporangia, 

 a condition seen constantly in the living Marattiaceae, and in most species 

 of Gkichenia. But in G. dichotoma y^ndpectmaia sporangia are found inserted 

 on the apex of the receptacle. They vary in number from one upwards, and 

 may form a second tier above the basal rosette. But all of them originate 

 simultaneously on the receptacle. As in other primitive Ferns there is no 

 succession in time. As the number 

 of sporangia is thus increased, the 

 available space is fully taken up so 

 that the sporangia will be in contact 

 laterally. The effect of this may be 

 seen in G. pectinata, where their 

 sides are flattened by mutual pres- 

 sure (Fig. 203). But the sporangium 

 oi Gkichenia opens by a longitudinal 

 slit, and elbow-room is therefore 

 required for the discharge of the 

 spores. The increase in number of 

 the sporangia will tend to prevent 

 this, and in G. pectinata the spo- 

 rangia are sometimes found undis- 

 - charged. Its sorus has reached the 

 practicable limit of proportion of 

 size and number of the sporangia 

 to the area of their insertion on the 

 receptacle. The mechanical dead- 

 lock may be met either by diminu- 

 tion of the size of the sporangium, 

 or by increase of the area occupied 

 by the receptacle, or by increase 

 of its vertical height. The first of 

 these is illustrated in G. dichoto^na, 

 which has relatively small sporangia; 

 but the other two methods have 

 been adopted by G. pectinata, which 

 marks in fact the limit of number of 

 sporangia per sorus attained in the family. In other related Ferns, however, 

 the increase in height of the receptacle, together with decrease in the size 

 of the sporangia, has resolved the difficulty, and led to the Gradate Sorus. 



The sporangia of the Ferns in which simple sori are present are relatively 

 i&w and large. The physiological point which they all have in common is 

 the simultaneity of origin of all the sporangia in near proximity to one 



Fig. 201. 

 W = part 



Ptychocarpiis unitus.. Fructification. 



of a fertile pinnule (lower surface), 

 showing numerous synangia. B, synangia in side 

 view. {A and B x about 6.) (After Grand'Eury.) 

 C—2L synangium in section parallel to the surface 

 of the leaf, showing seven confluent sporangia. 

 a = bundle of receptacle: b = \\.% parenchyma: 

 (r=tapetum: 6f= spores: £,/"= common envelope 

 of synangium. x about 60. (After Renault.) 

 (From Scott's Studies in Fossil Botany.) 



