no THE SPORANGIUM DEFINED 



protection and nutrition on the one hand, and ready dissemination on the 

 other. But the compromise may have worked out differently in different 

 lines of descent, and, indeed, it appears from the variety of the segmentations 

 in the Pteridophytes that this has actually been the case. From this point 

 of view no difficulty need be felt to arise from the absence of any general 

 law of segmentation, leading up to the formation of spore-mother-cells ; 

 but, on the other hand, similar and even definite types of segmentation, 

 culminating in regularly segmented sporogenous groups resembling one 

 another, may have been evolved along more than one line of descent. 



It cannot escape notice that in some cases the individuality of the 

 sporangium is not maintained. Certain synangial states are not uncommon, 

 which can only be regarded, from the evolutionary point of view, as 

 results of either septation or fusion : where the fusion or septation is 

 incompletely carried out, and partial septa are present, it becomes a 

 question whether the whole or only the part of the complex body is 

 correctly to be termed a sporangium. This difficulty is very obvious in 

 the sori of Danaea (Fig. 61). The mere application of a term is naturally 

 a trivial matter : the question which is really important is, how far the 

 conception of the sporangium is to be modified by the existence of such 

 cases. The current conception of the sporangium is based upon examples 

 where it has a distinct individuality : in the Ferns and Lycopods, and even 

 in the pollen-sacs of normal Angiosperms such individualised sporangia are 

 seen. But it is a question how far the idea of the individualised sporangium 

 so gained is an enlightening one from the point of view of descent. In 

 the same way, the old conception of the cell as the structural unit of the 

 plant-body was based upon the study of the tissues of the higher plants, 

 where the cells are for the most part individualised : it had to give way 

 before the accumulated examples of cell-fusions, of polynucleate cells, 

 and of non-cellular construction in plants both higher and lower in the 

 scale. Just as by comparison of such structures as these the idea of the 

 cell has undergone modification, notwithstanding that cells are commonly 

 definite bodies in the ordinary tissues of the higher plants, so may the 

 existence of evidence pointing to sporangial septations and fusions modify 

 the conception of the sporangium. 



The fact that sporangia originally simple have undergone septation has 

 only been proved in comparatively few of those cases. The most 

 complete demonstrations are those from the anthers of certain Angio- 

 sperms, such as the Onagraceae, Mimoseae, Loranthaceae, Rhizophoreae, 

 etc. In these the comparative argument is made valid by the existence 

 of numerous allied genera, which give ground for close comparison : for 

 while many plants of these orders show the ordinary quadrilocular anthers, 

 in others the loculi may be subdivided by further septa, and thus a 

 number of sacs take the place of each original one. The development 

 shows that the septation results from the conversion of sporogenous tissue 

 into sterile septa. Similarly, an argument for fusion of sporangia can also 



