ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 409 



thick walls. In exceptional cases the cells forming the conjunctive 

 tissue are smaller than the cortical cells (^Mimosa pudica). Crystals are 

 met with in the petiole in either the cortical parenchyme, the liber, or 

 the pith, and are of some taxonomic importance. They are either 

 (a) raphides ; (6) crystalline granulations ; (c) isolated crystals ; or 

 (d) quadrangular in shape. Various forms of secretory tissue may also 

 be found, and thick fibres often exist, especially in Malvaceae, Cruciferaa, 

 Umbelliferaa, and Compositte. Sclerenchyme is present either as 

 parenchymatous fibres or as sclerotized cells. 



As to the fibrovascular system : in the two peripheral bundles of the 

 petiole the phloem faces outwards, while the central bundles have a 

 variable structure. Bicollateral bundles are to be found in the petiole 

 of Solanacese, Convolvulaceae, Asclepiadeas, Apocynace^, Myrtaceae, 

 CucurbitacesB, and CEnotherea9. 



The author then gives a table illustrating the course of the bundles 

 in the petiole of Dicotyledons. The types may primarily be divided 

 into simple and complex, and the simple types are again divided into 

 those which have the bundles distinct both at the apex and base of the 

 petiole, and those where the bundles show the opposite arrangement. 

 Complex types are found in various natural orders, e. g. Rosacese, 

 Cupuliferae, Salicacese, &c. 



The most important results, however, the author gathers together in 

 a table at the end of the paper, in which he gives a resume of the dif- 

 ferential characters of the petiole in the principal families of Dicotyle- 

 dons. The primary divisions of this table are : — (A) the terminal section 

 in a petiole incloses secretory canals ; and (B) the terminal section in a 

 petiole does not inclose secretory canals. In division A we have 

 Umbelliferae, AraliacesB, Malvaceae, Tiliaceae Sterculiacese, and Compo- 

 sitae ; Umbelliferse and Araliacese have a secretory canal behind each 

 peripheric bundle, while the others have their secretory canals arranged 

 irregularly, and the hypodermal collenchyme discontinuous. In division 

 B we have Asclepiadese, Apocynaceae, Convolvulaceae, Solanacete, 

 Myrtacese, and Cucurbitacese, with bicollateral bundles ; and various 

 L'osacese, Malvaceae, Geraniaceae, Oxalideae, Cupuliferae, Amaranthaceae, 

 Chenopodiaceae, and Leguminosae, with no bicollateral bundles. Then 

 in the first five orders mentioned : — in division B the median bundle is 

 much developed ; Scrophulariaceae, Oleaceae, and Borragineee have the 

 lower bundle preponderating, and furthermore they have no scleren- 

 chyme. In Oleaceae the phloem is of more importance than in Scrophu- 

 lariaceae, where there are sometimes small prismatic crystals. Papa- 

 veraceae and Compositae possess secretory tissues ; Compositae have 

 ordinarily thick and sometimes even sclerenchymatous fibres, which are 

 never found in Papaveraceae. Cruciferae have thick fibres like Com- 

 positae, but no secretory tissue ; many Cruciferae can be recognized 

 immediately by the structure of their radiating bundles. Eanunculacefe 

 are distinguished by a transverse section of their fibrovascular bundles, 

 which have the form of an ellipse, in which the phloem is either circular 

 or elliptical. 



The author concludes by pointing out the great variation in structure 

 occurring in the petiole, also its importance for taxonomic purposes, and 

 the law which governs the general distribution of fibrovascular bundles 

 in herbaceous and woody plants ; isolation of the bundles generally 

 occurring in the former and aggregation in the latter. 



