72 PART I. THE MORPHOLOGY OF PLANTS. [ 16. 



The sporangia may be borne singly, or, as is more commonly the 

 case, in groups, each such group being termed a sorus. The spor- 

 angia of a sorus are generally quite distinct from each other ; but 

 in some cases (e.g. most Marattiacese, Psilotum, Tmesipteris) the 

 sporangia are coherent, forming what appears to be a multilocular 

 sporangium but is really a synangium. 



In those heterosporous plants in which the sporangia are in 

 sori, the two kinds of sporangia are borne in distinct sori ; the 

 only exception to this rule is afforded by the Marsileaceae, where 

 each sorus includes both microsporangia and macrosporangia. 



The sporangium is sometimes more or less completely covered 

 by a membranous investment to which the general term indusium 

 may be applied. In most Ferns {e.g. Hymenophyllaceae, Aspidieea, 

 AsplenieaD, etc.) and in Salvinia, the indusium covers a whole 

 sorus ; this is true also of the sori of microsporangia of Azolla, 

 and in some Cupressineae (Cupressus, Thuja, species of Juniperus) 

 the sorus of microsporangia is covered by an outgrowth which is 

 doubtless an indusium. On the other hand, when the sporangia 

 are borne singly, each individual sporangium may have its indu- 

 sium : this is the case with the macrosporangium of Azolla ; the 

 sporangia (of both kinds) of Isoetes, where the indusium is speci- 

 ally termed a velum ; the macrosporangia (ovules) of Phanerogams, 

 each of which is invested by one or two integuments, which are, 

 however, merely indusia. 



The mature sporangium of these higher plants is either borne 

 upon a stalk (sometimes termed fanicle) ; or it is sessile, and then 

 it is commonly more or less imbedded in the tissue of the parent 

 member, as in the case of the sporangia of the Ophioglossacese, and 

 of the pollen-sacs of most Phanerogams. This latter arrangement 

 is due to the fact that the growth of the adjacent vegetative 

 tissue keeps pace with that of the developing sporangium, so that 

 the sporangium does not stand out on the surface. 



The development of the sporangium begins,, in all cases, at the 

 surface of the parent member. The area which bears the spor- 

 angium, especially when a number of sporangia are developed 

 close together, generally projects more or less as a cushion of 

 tissue to which the term placenta is applied. In the Ferns (except 

 Marattiace83,0phioglossace8e, and Isoetacese) and in the Hydropteri- 

 deae (Rhizocarps), the sporangium is developed from a single super- 

 ficial cell : in the rest of the Pteridophyta and in the Phanerogams 

 it is developed from a group of superficial cells, and in some cases 



