154 PART II. THE INTIMATE STRUCTURE OF PLANTS. [ 31. 



layer of cells covering the body of the plant, and is commonly 

 termed the epidermis ; but neither the morphological nature of 

 this tissue, nor its structure, is uniform throughout the Vegetable 

 Kingdom. 



Morphology. A true epidermis only exists in those plants, and in 

 those parts of them, where there is a definite dermatogen ; a true 

 epidermis can, therefore, only be present in the Phanerogams and in 

 the roots of the Pteridophyta, in view of the structure of the grow- 

 ing-points as described in the previous paragraph, and the word 

 epidermis is, strictly speaking, only applicable to such a tegumen- 

 tary tissue. It is, however, convenient to apply this term gene- 

 rally to the primary tegumentary tissue of the shoot, and to apply 

 the term epiblema to the primary tegumentary tissue of the root, 

 apart from the root-cap. 



As regards the shoot, whilst there is a true epidermis in 

 Phanerogams, that of the Pteridophyta (as also of the lower plants) 

 is merely the external layer of the cortex. 



In the root there is considerable variety in the morphological 

 nature of the epiblema. Thus, in Hydrocharis and Lemna, the 

 dermatogen is and remains a, single layer of cells which persists 

 in the adult portion of the root as the epiblema which is, therefore, 

 a true epidermis morphologically equivalent to that of the stem. 

 In most cases, however, the dermatogen becomes many-layered ; 

 this persists at the apex as a root-cap, but gradually exfoliates on 

 the older parts behind the apex, either partially, so that a single 

 layer of it, the innermost, persists as the epiblema of the root (as 

 in most Dicotyledons, Gymnosperms, Lycopodium, Isoetes) ; or 

 entirely (as in Monocotyledons, and in Nymphaeaceae among Dicoty- 

 ledons), so that the epiblema is merely the external layer of the 

 cortex. The root of Pistia and Pontederia has no dermatogen, and 

 consequently no epidermis at all, the epiblema being the external 

 layer of the cortex : the root of these plants is, in this respect, 

 quite similar to the shoot of the Pteridophyta, etc., mentioned 

 above. 



Structure. In the great majority of cases the primary tegumen- 

 tary tissue consists of a single layer of cells ; but to this rule there 

 are several important exceptions. Thus, the epidermis of the 

 foliage-leaves of certain plants (e.g. Ficus, Peperomia, Begonia) 

 consists of two or more layers of cells. Similarly, the root-cap to 

 be found in nearly all roots is a many-layered epidermis. Again, 

 the aerial root of certain epiphytic plants (Orchids, Aroids) has 



