THE GREAT GROUPS OF PLANTS 247 



as pointing to the origin of Isoetes (by reduction) from 

 the Lepidodendrales. One of the most cogent objections 

 to this theory is the great amount of reduction which 

 must be postulated; moreover, Isoetes has no cone, 

 while most of the Lepidodendrales have. ' The absence 

 of secondary growth in thickness of the stem and of a ligule 

 on the leaves, combined with the possession of a biciliate 

 sperm, in Lycopodium, would tend to preclude its close 

 affinity with Isoetes. While certain features of sporophyte 

 anatomy {e.g., the possession of a ligule) suggest Selag- 

 inella, it seems difficult to accept a close relationship 

 between Isoetes and the Selaginellales, since the sperms 

 of the latter like those of Lycopodium are biciliate, while 

 those of Isoetes are multiciliate. The possession of multi- 

 ciliate, sperms and the structure of the archegonia suggest 

 affinity with the eusporangiate pteridophytes, and notably 

 with the Marattiales. 



Without going further into details which belong to a 

 more advanced and technical treatise than this, and 

 disregarding certain mooted points, or almost equally 

 balanced choices like the one just mentioned, it may be said 

 that the following tabular statement (pp. 249-251) reflects 

 the present state of our knowledge concerning the rela- 

 tionship and developmental sequence (phylogeny) of the 

 great Divisions and Orders^ of the Kingdom of Plants. 

 The same thing is shown diagrammatically in Fig. 112 

 (p. 248). The tabular statement aims, not only to indi- 

 cate the relationship and sequence of groups, but also to 

 help the student define the terms commonly met with in 

 the established literature of botany. 



1 Attention is called, in passing, to the uniform termination (-ales) of 

 the plant Orders. 



