FORM OF THE PLANT-CELL. 79 



tissue exhibits great elasticity, and the tissue known by the name of cork 

 belongs to this form. It exists, however, in countless other forms, and 

 its existence seems detetermined by the presence of an epidermis which 

 vegetates for a longer period than is usual. When the process of cork- 

 formation once commences, it goes on ; but should the layer be thrown off 

 the tree at any particular stage of its growth, it is not again engendered, 

 as, for instance, in the vine and the Clematis Vitalba. Mohl * was the 

 first who accurately examined this subject, and I have sought to explain 

 its origin, f 



Root-sheaths. If the organs of Pathos crassinervis, called aerial-roots, 

 are examined, there will be found a distinct epidermis, with stomates whose 

 semilunar cells, filled with a brown granular matter, are elevated above 

 the surface of the epidermis, and form a special tissue whose walls exhibit 

 the most delicate spiral fibres. These cells are filled with air, and thus 

 give the brilliant white appearance to these roots. How this layer origi- 

 nates is not very clear, but it is formed in the same way at the points of 

 the roots as in the other parts. The same layer is found on the roots of 

 most tropical Orchidacece, and the cell-walls exhibit in them the most 

 striking modifications. It is very remarkable in Aerides odoratum. I have 

 seen it in Epidendrum elongatum, Cattleya Forbesii, Brassavola cordafa, 

 Maxillaria atropurpurea, M* Harrisonii, Acropera Loddigesii, Cyrto- 

 podium speciosissimum, Oncidiunt altissimum, and other species. I also 

 found it, but without spiral fibres, in Pothos reflexa, acaulis, violacea, 

 cordata, longifolia, and digitata. In other families I have not seen it. 

 The roots have ordinarily a fresh green point ; in these the cells are full 

 of sap, and the green cortical parenchyma is seen through them. The 

 relations of this layer differ so much from those of the epidermal cells, 

 that it has been regarded as a peculiar tissue. Link J first discovered 

 this layer, Meyen examined it more accurately, but no one has correctly 

 appreciated it. 



* Ueber die Entwickelung des Korkes und der Borke. Tub. 1836. 

 f Beitrage zur Anatomie und Physiologie der Cacteen. 

 \ Elem. Phil. Bot. Ed. i. p. 393. 



Physiologie, i. p. 47. Meyen, copying Link, says that Dutrochet has examined 

 this tissue ; but this is a mistake. 



