THE MICROSPORANGIUM 215 



giving them an impetus which scatters them over the surface of 

 the ground. These spore-distributing mechanical cells are known 

 as elaters. In certain liverworts, such as Pellia and Aneura, 

 the elaters, in addition to occurring loosely among the spores 

 which they serve to scatter, are aggregated in a compact elon- 

 gated mass at one end or the other of the spore sac or theca. 

 This longitudinal cluster of elaters may be regarded with some 

 degree of probability as the prototype of the fibrovascular bundle 

 of the Pteridophyta and higher groups of vascular plants. In 

 Anthoceros and allied forms the cluster of elaters, known as the 

 columella, becomes a much more important structure and traverses 

 the sporangium from end to end. Laterally at intervals it gives 

 off transverse ramifications which divide the mass of spores into 

 separate clusters, and these may perhaps be regarded as the proto- 

 types of the sporangia found in the vascular series. The situation 

 in the horned liverwort Anthoceros, in which there is an extensive 

 columella with lateral ramifications, gives some support for the 

 hypothesis of the derivation of the sporophyte from the sporogonium 

 by the sterilization of potential sporogenous tissues. The mode 

 in which the organs, leaf, stem, and root arose from such a primitive 

 condition of organization is much disputed, since none of the hypo- 

 thetical transitional forms between the moss capsule and the sporo- 

 phyte of the higher plants have yet been discovered. There can be 

 little doubt, however, in a general way that the sporogonium is the 

 forerunner of the sporophyte and that the elater is the prototype 

 of the tracheid in vascular plants. 



It will be clear from the foregoing statement that the sporangium 

 is so intimately involved in the primitive organization of the most 

 ancient spore-producing members that it is entirely proper to 

 consider it a definite organ of higher plants on a footing of equality 

 with the root, stem, and leaf. If this view of the matter is sound, 

 obviously no very useful purpose can be served by the examination 

 of the development of particular cells of the foliar organs which are 

 in some way or other related to the origin of sporangia. It is 

 likely, moreover, that the sporangia of the Pteridophyta give us on 

 the whole a less accurate picture of the original type of sporangium 

 than those of the lowest seed plants. It is therefore appropriate 



