ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 6*59 



year, the outer cortex usually forms a colloncliymatous hypoclermal layer, 

 wliicli is not the case where the cork-layer is more internal, or at least not 

 to the same extent. lu Ulmus and Tilia the inner cortex contains mucilage. 

 The various special modes of development of the primary cortex are 

 then classified under 10 dilferent heads. In the first 6 the primary 

 bark consists of two and only two distinct layers. In the first 5 of these 

 the outer layer is distinctly collenchymatous. Of these again two types 

 have a homogeneous iuntr layer (^Sorhts, Crafsegus, Syr'mga vulgaris. 

 Viburnum Lantana, species of Itosn, most Salicacete and Betulaceee, and 

 others), while in 3 the inner bark is heterogeneous (Ulmus montana, 

 Tilia, Cnpuliferfe, Jiiglans). In the sixth type the outer cortex is not 

 distinctly collenchymatous (Vaccinium vitis-idsea, Azalea procumbens, &c.). 

 In the seventh the primary cortex is differentiated into more than two 

 layers (Leijcesteriay In the eighth it is differentiated into two layers 

 only in certain longitudinal strips (Viburnum Opuliis, Cornus sanguinea, 

 Forsythia, &c.) ; while in the remaining two there is no distinct 

 differentiation into more than one layer (Ehamnus cathartica, Prunus 

 spinosa, Eippopliae rhamnoicles, &c.). 



Pericycle.* — M. A. de Wevre states that before Van Tieghem the 

 pericycle was known under the name of pericambium or rhizogenous 

 layer. In the root the first indication of the pericycle was given by 

 von Mohl in 1831. (Tcnerally the pericycle is formed of a single layer 

 of cells (Rammculus, Veratrmn, &c.), less often of two layers (Vanilla 

 planifolia), or it may be of a larger number of cells (five to six in 

 Cynodon Dacfylon). It is said to be homogeneous when all the cells are 

 similar, and heterogeneous when it is composed of cells of various kinds, 

 as in Araliacefe, Umbelliferfe, and Pittosporefe. The heterogeneous 

 pericycle is most commonly met with. The author then points out 

 various modifications which the j^ericycle may undergo, and concludes 

 by stating that, in general terms, the root and the stem may be said to 

 be corcposed of three jn-incipal zones : — (1) The cortex, comprising all 

 the tissues up to the endoderm ; (2) the pericycle ; (3) the central 

 cylinder, composed of pith, wood, cambium, and liber. In the root a 

 portion of the secondary central cylinder is sometimes formed by the 

 pericycle. 



Mechanical System in the Roots of Aquatic Plants.f — M. C. 

 Sauvageau states that M. L. Olivier in 1881 established that in Mono- 

 cotyledons the endoderm and peripheral membrane of the central cylinder 

 are capable of thickening ; the thickening being a protection to the 

 liber-bundles. 



The central cylinder of the root of Naias major is composed of one 

 or two axile vessels, representing the xylem, surrounded by a variable 

 number of sieve-tubes of pericyclic origin separated from the central 

 bundle by conjunctive cells. No element of this central cylinder is ever 

 lignitied or suberized, all the walls remain white, and iodine and sul- 

 phuric acid colour all the cells blue. The radial walls of the endoderm 

 display elegant foldings. In the piliferous layer of the root the piliferous 

 cells alternate regularly with cells which do not produce hairs ; the 

 hairs have ultimately a singular collar near tLeii base. 



The central cylinder of the root of Naias minor is somewhat simpler 



• CR. Soc. Bot. Belsr,, 1S89, pp. 41-7. 



t Jouru. de Bot. (Morot), iii. (1889) pp. 3-11, 61-72, 169-81 (10 tigs.). 



