228 



PRINCIPLES OF PLANT-TERATOLOGY. 



G-audichaud, Schultz-Schultzenstein, Celakovsky, and 

 others, according to which the stem is bnilt up of a suc- 

 cession of segments each one of which is composed of a 

 leaf -blade and a leaf -base ; morphologically and primi- 

 tively, each leaf -blade terminates the whole growth, the 

 succeeding one sprouting from it in a lateral position. 

 The Monocotyledonous seedling shows us this primi- 

 tive type in actuality, and it is sufficient to contemplate 

 a typical mature Umbelliferous or almost any Mono- 

 cotyledonous plant to be satisfied as to the probable 

 truth of this theory. For the arguments in favour of 

 it the reader is referred to the literature of the subject 

 and to what has been said above. If true, the Mono- 

 cotyledonous type of stem, with alternate phyllotaxis 

 (i. e. the segmented stems with sheathing leaf-bases) 

 represents the primitive type, and all unsegmented 

 stems are derived therefrom. Both kinds of stem 

 occur in the Umbelliferse ; e. g. Domin describes 

 a typical genus, Didiscus, with unsegmented stem- 

 structure ; in a single individual of D. cyanope ialus he 

 observed a most interesting reversion to the original 

 segmented condition, for this plant possessed through- 

 out leaves with sheathing bases completely enclosing 

 the stem. 



The mode of development of the earliest-formed 

 foliar organs of the seedling has generally been regarded 

 as affording a clue and an indication as to the nature 

 of the foliage-leaves in the ancestors of the plant con- 

 cerned. This is one of the comparatively rare instances 

 in which the ontogeny is of real assistance in the un- 

 ravelling of the thread of the phylogeny. In this 

 connection our attention must be directed not only to 

 the conformation of the mature cotyledon or first- 

 formed foliage-leaf, but also to the stages in the onto- 

 genetic history of these organs. Now, in those plants 

 possessing sheathing leaf -bases, the first part to be 

 formed in these early foliage-leaves is the sheathing 

 base, and only at a later stage do the petiole and lamina 

 become differentiated, while in many plants the first- 



