74 CELLULAR TISSUE. 



the epidermis far from the inner tissues of the leaf, and appears itself like a special 

 mucilaginous layer of tissue. 



The same phenomenon, but apparently always developed to a less extent, was 

 found by Radlkofer in the foliage leaves of numerous Dicotyledons ; e.g. Sapindaceae, 

 species of Salix, Daphne, Quercus pedunculata, Betula alba, Erica carnea and 

 Tetralix, species of Prunus, Genista, Cytisus spec, &c., of Ferns in Botrychium 

 Lunaria. As shown by Radlkofer's comparative review of the cases investigated, 

 the phenomenon is by no means generally distributed, nor is it generally peculiar to 

 definite forms of leaf or systematic groups ; it is absent, e.g. in Salix alba, amygdalina, 

 Betula fruticosa, Prunus Padus, &c. In the Sapindaceae investigated by Radlkofer 

 it is often only single cells, or groups of cells, which show the phenomenon 

 in que.siion, 



Cu/i'n occurs in form of the cuticle^ and in the cuticidar layers of the cellulose 

 membrane \ 



Cutin is a non-nitrogenous carbon compound, which is completely combustible ; 

 it is dissolved, or destroyed by boiling solution of potash, and by Schultze's mixture ; 

 it can therefore be completely removed from the epidermis by these reagents. It is 

 only slightly attacked by mineral acids, especially sulphuric acid, which destroy 

 cellulose. It remains unaltered in ammoniacal sub-oxide of copper after previous 

 treatment with acids ; the same is the case with water, alcohol, and ether. It resists 

 rotting far longer than cellulose. By these reactions the means are supphed for 

 isolating the cuticle and the cuticular layers. Iodine preparations, with or without 

 the assistance of sulphuric acid, colour the cuticle and cuticular layers yellow or 

 brown. Aniline dyes are quickly taken up by them in large quantity, and deep 

 coloration is the result. From a mixture of aniline red and violet, the latter is often 

 (not always) taken up more abundantly 2. 



The cuticle covers the whole outer surface of the epidermis, including the hairs, 

 as a thin, always closely applied hyaline skin. It appears, secreted on the outer surface 

 of the cellulose walls, on the embryo while it consists of only few cells, and covers it 

 henceforward as well as the punctuvi vegeiationis of the stem and all the members 

 which appear on it, always following the growth of these by means of corresponding 

 surface-growth, while its increase in thickness is infinitely less. This continues till 

 the epidermis is eventually thrown off. In rare cases, when the original epidermis 

 is destroyed early, and is then replaced by new elements (the leaves of the Aroideae 

 mentioned on p. 29, and perhaps also of Palms), there appears also over the latter a 

 new cuticle. It is wanting at the puncitim vegetalionis of the root, but appears 

 behind it, where the differentiation of the epidermis begins. The cuticle is con- 

 tinuous over the surface of the guard-cells, through the stoma, into the respiratory 

 cavity, from the moment when the formation of the slit begins by separation of the 

 two cellulose lamellae. It usually continues over the walls of the respiratory cavity 

 as far as these are formed from the epidermal cells. It becomes gradually thinner 

 as it proceeds inwards, till it ceases where the respiratory cavity is laterally bounded 

 by subepidermal cells. It thus forms at each stoma an open tube which passes 



' [Cf. Von Hohnel, iiber die Ciiticula. — Rcf. Bot. Jahresbericht, 1878, I. p. 16.] 

 ^ Hanstein, Botan. Zeitg. 1868. 



