EPIDERMIS, 



87 



show that they correspond to deep tube-like depressions of the surface, but these are 

 not complete perforations : it must remain undecided whether such perforations may 

 occasionally occur. The three constituents are unequally distributed in correspondence 

 with the longitudinal bands with and without stomata, which alternate in the leaf- 

 surface. On the former the holes are very near together, and numerous: the spots 

 between them form a narrow very irregular net, the substance of which has chiefly the 

 darker, blueish appearance. The network is densest and the meshes narrowest over 

 the stomata themselves, i.e. over the pair of guard-cells: a narrow slit-like hole 

 corresponds usually, but not always, to the slit of the stoma. On the bands of the 

 leaf without stomata, the wax-layer is more homogeneous : it has more solitary usually 

 slit-like holes. 



Corresponding to these appearances of the surface view, the transverse section of the 

 wax -layer shows straight bands, of alternating unequal refractive power, which are 

 parallel and perpendicular to the surface. The clearer parts correspond partly to 

 the holes, partly to the more transparent fundamental mass. On the parts which have 

 many pores and stomata, the striation is, as might be expected, much denser and more 

 clearly defined than on the parts with no stomata. But figures, like Wiesner's Fig. b, I 

 have never seen, at all events not on clearly cut sections. It must finally be added to 

 the above typical description, that the regions distinguished are not always quite sharply 

 limited from one another. Again, Wiesner's description of the wax of the sugar-cane 

 corresponds so imperfectly with the appearances found on fresh plants, that the idea 

 suggested itself of different structure of the covering in different sorts or varieties. My 

 observations on material, also kindly supplied by Wiesner, on which his statement was 

 founded, do not confirm me in such an assumption. The traces of dense, normal rod- 

 structure may be clearly recognised, and are made indistinct by all sorts of injury 

 (especially very many fungal hyphae are present), just as one may observe it on the old 

 nodes of living plants (comp. Bot. Ztg., /. t., p. 151). Wiesner's statement that the wax 

 layers and rods appear doubly refractive under polarising apparatus is quite correct ; the 

 same holds for wax-deposits, which I investigated on this point : my only reason for not 

 mentioning this earlier, was that it appeared to me to afford no decided conclusion 

 concerning the nature of the layer. 



As regards the development of the wax-layers, it is proved that they appear on the 

 persistent cuticle, and that they are extruded, and do not owe their origin to a meta- 

 morphosis and metacrasis of the cuticle and cell-membrane itself, as earlier writers 

 had asserted (Wigand, Karsten, Uloth). Wax-layers may arise from very dense simple 

 granular coverings, by lateral fusion of the granules as they increase in number and size. 

 The typical layers arise in the first place as such : they may follow, by intussusceptive 

 growth, the increase of girth of the part which bears them (Kerria, Euphorbia, Chamae- 

 dorea). The coverings appear, either when the part which bears them is very young 

 (Eucalyptus, Acacia, Dianthus, Echeveria, Carnauba palm), or only in later stages 

 of development, e. g. not till the unfolding of the bud (leaves of Strelitzia, Heliconia, 

 Galanthus, Tuljpa, Cotyledon orbiculata, stem of Saccharum, Chamasdorea, fruits of 

 Myrica, Benincasa). Cases of the latter sort should be chosen for studying the 

 development. The extruded wax may first be determined in the cell-wall, or cuticle, 

 and emerges from the latter on the surface : in no case can a trace of wax be found 

 previously formed in the interior of the cells which secrete it, so as to be transmitted 

 outwards through the niembrane. 



The granular and rpd-like coverings form that bloom or rime on surfaces which is so 

 easily wiped off. After it has been wiped off it can be renewed ', provided the part of 

 the plant has not passed a certain stage of development, which must be specially 

 determined for each special case. As far as experience goes, the renewed covering has 

 the same structure as the primary one, but is less massive. 



' De CandpUe, Physiol, p. 233.— Treviranus, Physiol. II. p. 44- 



