220 SCIENCE PROGRESS. 



Flot's "vascular meristem," i.e., the ring of tissue produc- 

 ing the bundle system plus the "external conjunctive": 

 shortly became differentiated from the peripheral zone or 

 young cortex. In other cases {Euonymus and Berberis), 

 the "thickening ring" appeared or began to appear before 

 the young pith became separated from the " outer zone". 

 Hanstein (25), as a consequence of his separation of the 

 primary meristem into Dermatogen, Periblem and Plerome, 

 makes the outer limit of the young cylinder, i.e., that 

 between periblem and plerome, of primary rank. Russow's 

 scheme (26), on the other hand, drawn from instances like 

 those of Sanio's first group, 1 in which the young pith is the 

 first tissue to become apparent, divides the young tissue 

 produced by the general Protomeristem at the apex itself 

 into Endistem (Sanio's young pith) and Existent (Sanio's 

 " Aussenschicht "), the latter being separated into Mesistem 

 (Sanio's "thickening ring ") and Peristem or young cortex. 

 Thus the limit between "Mesistem" and "Peristem' is 

 reduced to secondary rank. But De Bary (14, pp. 395-6) 

 again sums up clearly in favour of the individuality of the 

 plerome. 2 As a matter of fact the young pith often does 



1 Russow placed Hanstein's best instances, for example, stem of Hip- 

 puris, and Roots, where there is a well-defined plerome at the apex itself, 

 under the separate heading of "Axes with Combined Bundles ". 



2 The development of the pericycle is of great importance in this 

 connexion. Sanio (24) showed in several cases that what we now call the 

 pericycle was developed from the outer edge of the "thickening ring". 

 Schmitz (27) confirmed this view in Berberis and Menispermum. Van Tieg- 

 hem, however (5), based his conception of the pericycle entirely on the 

 ground of adult comparative anatomy. This is explicitly stated (p. 152) in 

 a remark he made at the close of a " Note sur le pericycle," read by 

 D'Arbaumont (28) to the Botanical Society of France. D'Arbaumont had 

 endeavoured to show that the sclerised portions of the pericycle, capping 

 the phloems of the stem bundles in dicotyledons, were developed in 

 common with the bundles themselves from the desmogen strands, and 

 were thus often separate from the interfascicular pericycle. His account 

 of the development of the continuous zone of fibres in Cucurbitacese and in 

 Berberis is different, and indicates differences in the origin of the pericycle 

 in various plants. It is unfortunate that no figures are given. Morot re- 

 plied (29) that even if the pericycle, or parts of it, were developed dif- 

 ferently in different plants, that made no difference to the validity or applica- 



