126 VARIATIONS IN NUMBER OF SPORANGIA 



(d] Closely related to (c) is the branching of parts bearing sporangia : 

 this may also occur in parts which are in no way morphologically com- 

 parable. Branching of strobili is a common feature in Lycopodium and 

 Psilotum : branching of the sporophyll is characteristic of most large-leaved 

 Pteridophytes : branching of the receptacle of the sorus is common in 



Ferns (Fig. 68) : branching of the fertile spike 

 is a marked feature in certain Ophioglossaceae : 

 branching of stamens is common in the Angio- 

 spermic flower, and is to be distinguished from 

 the interpolation above mentioned : a somewhat 

 similar branching of the sporangiophores has 

 probably occurred in the Sphenophylleae. In 

 all such cases, though the parts are not mor- 

 phologically comparable, the end is attained 

 of an increased accommodation for sporangia, 

 which consequently may be produced in 

 increased numbers. 



(e) Continued growth and branching, occur- 

 ring in the non-sporangial region, may have 

 the indirect effect of a further increase in the 

 opportunities for production of sporangia; for 



not only is the vegetative system thereby increased, which will have its 

 indirect effect in increased powers of nutrition, but also a larger number 

 of apices are provided, any one of which may take up the character of a 

 sporangial strobilus. The continued apical growth in the vegetative region 

 is a general feature of Vascular Plants. Branching is profuse in many of 

 the strobiloid Pteridophytes : in the Ferns also it occurs, but the effect in 

 increasing the opportunities for spore-production is less obvious here than in 

 the strobiloid types. In Flowering Plants also the complicated inflorescences 

 and the multiplicity of flowers is dependent upon such apical growth, 

 together with repeated branchings. 



FIG. 63. 



a-d, various examples of fission of 

 the sorus in Hymenophyllum dila- 

 tatum. The sporangia, and one flap 

 of the indusium have been removed, 

 leaving the receptacle exposed. 

 X about 5. 



FACTORS OF DECREASE. 



(/) Decrease in number of sporangia, by fusion of sporangia which 

 previously in the race were separate, has been assumed as an explanation 

 of synangial states by various writers : but it can only rarely be proved on 

 grounds of comparison that fusion of sporangia has actually taken place, 

 and the best evidence of it comes from the Angiosperms. Thus the fusion 

 of the ovules, leading indeed to the obliteration of their identity, occurs in 

 certain species of Loranthus, and comparison leaves little doubt that the 

 sunken embryo-sacs represent the individual ovules, the identity of which- 

 is lost as regards external form. Fusion of pollen-sacs is more frequent ; 

 good examples, showing various states of the fusion, are found in the 

 genus Phyllanthus, and in Cyclanthera while the unilocular condition in 





