166 MANUAL OF BOTANY 



place in the archesporium then a process of steriUsation of 

 certain of these cells, recalling the formation of the elaters in the 

 Liverworts. The sterile cells extend as trabeculae or strands 

 across the interior of the sporangium, so that in transverse 

 section it appears to be septate. In the microsporangimii other 

 cells also sterile, though derived from the archesporium, form a 

 tapetum, which surronnds the mass of microspores. In the 

 macrosporangium there is a similar formation of trabeculae, but 

 the arrangement of ihe rest of the archesporial tissue is different. 

 There are fewer cells, which are consequently larger, and a tape- 

 tum is formed round each mother-cell. 



The sterilisation of the tissue forming the trabeculae, like that 

 of the tapetum, is probably due to the need of distributing 

 noiudshment for the spores throughout the large sporogenous 

 mass. 



The gametophytes of Isoetes closely resemble those of 

 Selaginella. The prothallium derived from the macrospore is 

 even more completely endosporous, the development being 

 advanced to the stage of maturity of the archegonia before the 

 spore splits. The prothallium consequently never becomes 

 green. The antheridia and archegonia resemble in all points 

 those of Selaginella. 



The development of the young sporophyte from the zygote 

 recalls that of the ferns. Both epibasal and hypobasal segments 

 divide to form the octants ; the hypobasal ones all combine to 

 form the foot, but the first root is developed from the epibasal 

 segments, which also give rise to the stem and the single 

 cotyledon. The root is consequently adventitious, as in 

 Selaginella. 



Like the Equisetinae, the Lycopodinae were represented in 

 Carboniferous times by very massive forms which showed great 

 cambial activity in the development of their trunks and roots. 

 Of these Lepidodendron is the best known example. 



