RECENT WORK, ETC. 477 



from the pith itself. The cells from which the central 

 cylinder is derived, divide by lateral (periclinal) walls to 

 form the " vascular meristem," and by walls at right angles 

 to the axis (anticlinal) to form the true pith, or inner 

 conjunctive tissue. The " vascular meristem ' of Flot 

 gives rise to the vascular bundles and to the outer con- 

 junctive tissue, which includes the pericycle, the primary 

 rays, and the perimedullary zone. 



Where the bundles remain separate, the perimedullary 

 zone only becomes differentiated on the inner side of each 

 individual bundle ; where they are united to form a con- 

 tinuous rino- this zone forms a continuous rino- also. It 

 may become wholly or partially converted into sclerenchyma, 

 just as is the case with the pericycle, and like the latter may 

 be the seat of various new formations. Thus the internal 

 phloem which occurs in Cucurbitacese, Solanaceae, and many 

 other Dicotyledous orders, is derived from the peri- 

 medullary zone, and so is the internal wood and bast found 

 around the pith in species of Tecoma, Rumex, etc. It 

 appears therefore that internal phloem belongs neither to 

 the vascular bundles themselves, nor to the true pith ; it 

 arises from this intermediate zone, which had not been 

 distinguished by previous observers. 



Flot's figures appear to show that his perimedullary 

 zone or internal pericycle, is really distinct, differing from 

 the pith both in its origin and in the form of its cells. 



Van Tieghem, in his Traitt de Botaniquc (5, p. 763), 

 calls special attention to stems with perixylic structure, or 

 in other words, with centripetally developed primary wood. 

 Perixylic structure is characteristic of all roots, and of the 

 first stage of the stem (hypocotyl) in most vascular plants ; 

 it persists throughout the whole of the stem in most 

 vascular Cryptogams, while centroxylic structure (centrifugal 

 primary wood) is general in the epicotyledonary stem of 

 all Phanerogams, and of some vascular Cryptogams, as 

 Equisetuni and the Ophioglossece. This distinction is 

 certainly one of the most important in anatomy, and it 

 appears probable that perixyly is the more primitive 

 condition. The peculiar structure of the foliar bundles in 



