THE SPORE-PRODUCING ORGANS 



ICH. 



another. The effect of this is that the physiological drain of spore-production 

 is imposed simultaneously by all the sporangia upon the part which bears 

 them. Such Ferns as shozv the siviultaneotis scheme have been styled the 

 SiMPLicEs, and the}' include the Botryopterideae and Zygopterideae, the 

 Marattiaceae, Osmundaceae, Ophioglossaceae, Schizaeaceae, Gleicheniaceae, 

 and Matonineae. These are all primitive types, and many of them have an 

 early history as fossils. It may, therefore, be concluded that this condition 

 of the sorus is itself prijiiitivc. 



Fig. 203. A single sorus of Gleichenia pectiiiata, 

 showing the sporangia so closely packed as 

 to be flattened against one another. One has 

 already dehisced. Note the inverted position 

 of one of the outer sporangia on the right- 

 hand side. Enlarged. 



Fig. 204. Hymcnophylhivi Wihoni, 

 Hk. Sorus in longitudinal sec- 

 tion, showing the receptacle with 

 divisions indicating intercalary 

 growth, and tlie first sporangia 

 [s) originating near to the apex, 

 (xioo.) 



The Gradate, or Basiretal, Sorus 



The device of increasing the area of the soral receptacle has been fully 

 made use of by the Marattiaceae, and they have thereby increased the pro- 

 ductiveness of the individual sorus, while still maintaining its simple type 

 (Fig. 202). But a similar advantage may be gained by increasing the height of 

 the receptacle, thereby affording greater accommodation in a different way 

 for a larger number of sporangia. Moreover these need not be produced 

 simultaneously but in succession. Naturally a basipetal succession will be the 

 most practical, so that the youngest shall be nearest to the source of nutrition, 

 while the oldest will be most exposed, and shedding of the spores eas}'. The 

 drain of nutrition will by this means be spread over an extended period. 

 This is the ph\'siological ratioiale of the Gradate Sorns. The receptacle, 

 which may be cylindrical (Hymenophyllaceae), or laterally compressed 

 {Cibotinm, Thyrsoptcris), retains the power of intercalary growth (Fig. 204). 



