106 PART II. ANATOMY AND HISTOLOGY. [ 30 



(Marchantiacese, Anthocerotacese) : the growing-point of the sporophyte of 

 the Liverworts has a group of four initial cells, whilst that of the Mosses 

 has a single two-sided apical cell. 



In the higher Algae, the shoot (or thallus) also grows by means of a 

 single apical cell : in the more filamentous forms (e.g. some Floridese, 

 Characeee) the apical cell is hemispherical in form, and segments are only 

 cut off by transverse walls parallel to the flat base; in other more bulky 

 forms of Floridese there is a group of initial cells ; in nearly all these forms 

 a more or less distinct differentiation of a central medullary tissue and of a 

 cortical tissue takes place : in the Fucacese there is a single apical cell in 

 the growing-point, with either three or four flanks, along which the seg- 

 ments are cutoff; when the apical cell is four-sided, segments are also cut 

 off internally along the truncate base of the cell; the tissues soon show 

 differentiation into a cortical and a medullary region. 



In unseptate or imperfectly septate plants, having apical growth (e.g. 

 Siphonaceous Algse, CJadophora, etc.), the growing-point (like the rest of 

 the body) is not cellular, but consists merely of embryonic protoplasm. 



30. The Primary Tegumentary Tissue. The primary 

 tegumentary tissue may be generally described as the external 

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

 termed the epidermis. 



Morphology. A^rwe epidermis only exists in_tho^ej)lants, and in 

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

 word epidermis is, strictly speaking, only applicable to such a 

 tegumentary tissue. It is, however, convenient to apply this term 

 generally to the primary tegumentary tissue of the shoot, and to 

 apply the term epiblem to the primary tegumentary tissue of the 

 root, apart from the root-cap. 



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 

 a many-layered epiblem, known as the velamen, consisting of 

 empty tracheidal cells with reticulated and perforated walls (see 

 p. 93). 



The cells of the epidermis of the shoot of land-plants, are 

 characterised by the thickening and cuticularisation of their cell- 

 walls (see p. 76). The external wall is usually much more thickened 

 than the other walls ; its outermost layer, termed the cuticle, is 



