142 



THEORY OF THE STROBILUS 



sterile central columella : such was probably the case also in the predecessors 

 of the strobilus, but the process was more completely carried out, so that 

 the spore-formation came to be, as it now is in all Pteridophytes, located 

 close to the outer surface. A further step would be the disintegration 

 of the sporogenous tissue into- separate pockets or sporangia: of such 

 disintegration there is evidence in certain Pteridophytes, but it is exemplified 

 in the clearest way in the anthers of various Angiosperms : the condition 

 which is actually seen in the anthers of Viscum album, or in the large 

 multilocular anthers of Rhizophora illustrates the point (Fig. 72): here 



the numerous, small, isolated loculi 

 cover the very considerable surface 

 of the enlarged stamen, and develop- 

 ment as well as comparison points to 

 an origin of these by segregation from 

 the normal type of pollen-sacs. The 

 outgrowth of appendages by enation, 

 from such an apically growing struc- 

 ture has been already recognised as a 

 probable feature ; if this took place 

 either between the segregated loculi 

 or below them so as to carry them 

 outwards beyond the general surface, 

 during its acropetal development, the 

 result would then be a strobiloid 

 structure with an acropetal succession 

 of appendages, such as is seen in 

 various Pteridophytes. Sometimes the 

 sporangia might be borne in close 

 relation to the axis, as in Lycopodium 

 or Selaginella, while other Lycopods 

 illustrate varying degrees of the carry- 

 ing of the sporangia outwards upon 



the appendage. In other cases varying numbers of sporangia are borne 

 upon a single appendage, as in the Calamarians and Sphenophylls : and 

 according to their form, and their relation to or freedom from sporangia 

 divers ranks of these appendages may be distinguished : these matters will 

 be discussed in detail later. 



In the hypothesis thus sketched there are several steps which may be 

 named as distinct, though actually they may have overlapped : they are 

 (i) the differentiation in the primitive sporophyte of a vegetative base,. 

 and a fertile upper region having a power of apical growth : (2) the 

 relegation of sporogenous cells in the latter to a superficial position : 

 (3) the segregation of them into separate pockets or sporangia : (4) the 

 enation of the appendages. Every one of these steps has its actual prototype 

 among living plants, so that nothing is advanced which is contrary to 



FIG. 72. 



Rhizophora mucronata. Flower in longitudinal 

 section. Numerous spherical microsporangia, />, 

 in the anther. (After Goebel.) 



