488 BOTANY OF THE LIVING PLANT 



have originated from a post-sexual production of spores, and have been 

 originally without vegetative tissue : the latter being produced in the first 

 instance by Sterilisation of some of the potential spore-mother-cells. 

 Comparison among the sporogonia of Liverworts and Mosses makes it appear 

 probable that such steriUsation has actually occurred in them. On this theory 

 the simple sporogonium of Riccia (Fig. 313), which has no distinction of ape.x 

 and base, would represent a primitive state, and more elaborate capsules may 

 be seriated progressively in relation to it. 



A good case can be made out for an alternative view ; that the Arche- 

 goniatae sprang from some Algal ancestry which already possessed alternate 

 but separate generations, of which the sporophyte developed from a freely 

 shed ovum. That on adopting a land-habit the ovum, ancestrally fertihsed 

 as it lay free in the water, was retained on the parent gametophyte, in a pro- 

 tective organ, the archegonium ; and that the Encapsulation had its effect 

 in moulding and modifying the young sporophyte to such forms as are seen 

 in the simpler Bryophytes. In the present state of actual knowledge such 

 suggestions are httle better than speculations. 



The sporogonium of the Bryophyta is usually held to represent 

 the most primitive type of sporophyte among Land-living Plants, 

 and recent discoveries of very early fossils tend to support that 

 opinion. Its limited plan of construction, and its ephemeral character, 

 the absence of appendages, the preponderance of spore-production, 

 and its dependence throughout life upon the gametophyte, are all 

 indications pointing in the same direction. The fact that the spore- 

 production is simultaneous in each sporogonium, and that it arises 

 from one undivided sporogenous tissue, not from a number of 

 distinct sporangia, also points to the same conclusion. Within the 

 Bryophyta the various forms may be seriated so as to give 

 probable indications of progressive advance in various important 

 characters. These suggest that from a very primitive spherical type, 

 such as Riccia (Fig. 313), apex and base were first defined. A sterile 

 stalk and central columella were acquired, and later, specialised 

 methods of spore-distribution. These rose to high efficiency in the 

 higher Mosses (Fig. 296). Incidentally, parts of the tissue of the 

 more complex forms assumed a Photo-synthetic function. A well- 

 formed epidermis and stomata are found in some of them, while 

 beneath this lie assimilating tissues as well ventilated as in the leaves 

 of Vascular Plants (Fig. 305). Nevertheless the simple form, the 

 limited apical growth, the absence of appendages, and above all the 

 want of any direct connection with the soil, stamp even the most 

 elaborate sporogonium as an only partially efficient structure. To 

 achieve higher development it would be necessary to break away 

 from so restricted a plan, which in itself is only practically possible 



