120 



ON THE PLANT-CELL. 



The cuticula (6) is remarkably thick in the Tree Carnation 

 (baumnelke) (fig. 105.), in which the first and firmer secretion can also 

 be clearly distinguished from the subsequent and softer deposit. 



105 



105 



In Cycas revoluta (fig. 106.) the entire secreted layer is homogeneous ; 

 but in this plant an interesting condition is presented, the epidermis- 

 cells exhibiting pores on the external wall, in consequence of which it 

 is more easy to distinguish the membrane of the epidermis-cells from 

 the secreted layer. 



Hugo Mohl * has furnished a whole series of other peculiar modes in 

 which this secreted layer is formed. 



Sometimes the first secretion is formed more prominently in definite 

 situations ; for instance, .on the middle of the cell (P/tormium tenax~), 

 or at two or three points, or at the margins of the stomates (Agave 

 americana), constituting warty and other similar productions. It 

 is frequently deposited so irregularly that it appears as if scratched 

 with needles, as in Epidendrum elongatum. In most cases the secretion 

 manifestly differs in aspect from the outer wall of the epidermis-cells. 

 Frequently the wall of the cells merely appears to be thickened ; but, 

 even in this case, careful maceration will render the secreted layer 

 evident, which is elsewhere readily seen. It is in this way that the 

 membrane termed cuticula by Brongniart is obtained, f Along with 

 this secretion, that of the waxy substance is also probably developed ; 

 for the more brilliant and less pervious to water, and the less readily 

 deprived of that property by means of alcohol, do we find the epidermis 

 cells to be in proportion to the greater thickness of the last-described 

 layer. 



I must here, however, mention two different views that have been 

 more lately advocated respecting the layer of secretion on the epidermis. 

 The former has been developed by H. Mohl in the Linnsea (1842). He 

 is of opinion that the secreted layer is wholly formed from the outer 

 walls of the eprdermis-cells, which become thickened in the usual lami- 

 nated manner, and, in fact, in such a way that normally the innermost 

 last-formed lamina acquires the nature of the original membrane, whilst 

 the exterior older lamince become gelatinous, or otherwise variously 

 modified by the common membrane. This view is based upon very pre- 

 cise and comprehensive investigations of the perfect epidermis, to which 



* Linnaea, 1842. 



f Annales des Sciences natur. torn. xxi. 



105 Perpendicular section through the epidermis of the leaf of a Tree Carnation, 

 e, Epidermis-cells covered with the secreted layer (fe), which, externally, is constituted 

 of a more dense deposit, cr, Passage through the secreted layer to the stomate. 



106 Section perpendicular to the surface of the leaf of Cycas revoluta. The epidermis- 

 cells (6) are porous on the lateral and external aspects, and, on the latter, covered with 

 the secreted layer. 



