260 SUMMARY OF CURRENT RESEARCHES RELATING TO 



and continuous, localized at the periphery of the central cylinder, and 

 separated from the bundles by several layers of fundamental tissue. 



Commenting on this paper, M. L. Morot * maintains that both the 

 pericycle and the pith may be entirely parenchymatous or entirely 

 sclerenchymatous, or partly the one and partly the other ; in the last case, 

 the sclcritication may be bounded at the outer or inner border of the 

 vascular bundles, or may form a continuous zone, or may present a more or 

 less irregular appearance. 



Development of Tracheides.j — Herr L. Kny discusses the question 

 whether elongated fibriform tracheides are, like vascular tracheides, de- 

 veloped out of a single cambial cell, or from a fusion of a number of cells. 

 In Coniferfe, and in many Dicotyledones the former is certainly the case, but 

 in many Monocotyledones with secondary growth in thickness, e. g. Aloe, 

 Yucca aJoifoIia, Dioscorea convolvulacea, Dracsena Draco, and Aletris fra- 

 grans, he was able to determine that they were the result of the coales- 

 cence of several superposed cambial cells. In some instances, they are from 

 26 to 30 times the length of an ordinary cell, and contain at first as many 

 nuclei as that number of cells of which they are composed. The ends of 

 such fibriform tracheides are completely closed. 



Central Cylinder of Stem, j — MM. P. Van Tieghem and H. Douliot point 

 out that in the cases described by de Bary as concentric bundles in which 

 the xylem is internal and the liber external, the concentric bundles really 

 consist of several central cylinders, resulting from the ramification of a 

 single central cylinder. The disposition of the vascular bundles in the stem 

 may be arranged under three types, viz. (1) A single central cylinder (mono- 

 stelic structure) ; this includes all roots except those of Lycopodiace^, the 

 greater number of the stems of Plianerogams, the petiole of Solanaceae, 

 Cucurbitacete, &c. ; (2) Several central cylinders (j)oli/stelic structure) ; 

 including the stem of species of Auricula and Gunnera, the greater number 

 of ferns, Marsiliaceae, Selaginellacese, and Lycopodiacese, the petiole of many 

 ferns, and the root of Lycopodiaceas ; (3) Vascular bundles isolated, without 

 any central cylinder {astelic structure) ; the stem of Nymphteacese, of 

 Hydrocleis, and of several species of Ranunculus ; the lamina of fern-leaves. 

 Structure of Crassulacese.l — According to M. H. Douliot, the stem and 

 roots of Crassulacefe are normal in their primary structure ; but in the stem 

 secondary formations occur, giving the appearance of a " polystelic " 

 structure |1 when the concentric foliar bundles increase in size. The same 

 may take place in the root from divisions of the generating layer in several 

 arcs ; but the modification is here again secondary, and the " polystelism '•' 

 only illusory. 



Anatomy of Casuarineae.^ — M. H. Lecomte has examined the anato- 

 mical structure of several sjiecies of Casuarina. He agrees with the pre- 

 valent view that the longitudinal ridges on the stem are of the nature of 

 decurrent leaves, as is shown by their possessing a palisade-parenchyma 

 and a special fibrovascular bundle in each rib. A transverse section of a 

 young branch presents two concentric circles of vascular bundles, the outer 

 of which belongs to the leaves, the inner to the central cylinder of the 

 stem, its bundles being alternate with those of the outer circle. The 

 course of the vascular bundles presents a striking analogy to that which 



* Bull. Soc. Bot. France, viii. (188f3) pp. 203-6.1 



t Ber. Deutsch. Bot. Gesell., iv. (1886) pp. 267-76 (1 pi.). 



X Bull. Soc. Bot. France, viii. (1886) pp. 213-6. 



I Il)icl., pp. 299-306 (7 figs.). 1| See preceding note. 



T Bull, teoc Bot. Fiance, viii. (1886) pp. 311-7. 



