35.] CHAPTER II. THE TISSUES. 211 



In other cases (e.g. stem of Poterium, Alchemilla, Agrimonia, 

 Epilobium) the periderm consists mainly of cells with cellulose- 

 walls, between which intercellular spaces are formed, together 

 with occasional compact layers of cells with a suberised zone. 



It should not be overlooked that the regeneration of the root-cap is a case 

 of development of secondary tegumentary tissue, though of a different type, 

 inasmuch as it is effected by the primary meristem of the growing-point of the 

 root instead of from a layer of secondary meristem as is the case with the peri- 

 derm. As the root elongates and makes its way through the soil, the primary 

 root-cap becomes worn away at the surface, but this is compensated by the 

 development of new tissue internally (see p. 158). In some aquatic plants (e.g. 

 Lemna, Hydrocharis, Pistia, etc.), in which the root does not penetrate the 

 soil, the primary root-cap persists. In the case of roots having limited growth 

 in length (e.g. Azolla, Hydrocharis, tuberous roots of Orchis and of Ranunculus 

 Ficaria), the root-cap is altogether thrown off when the growth in length of the 

 root comes to an end. 



When the primary periderm is of superficial origin, it forms for 

 many successive years the external investment of the branch ; it 

 may attain considerable thickness, as in the Cork-Oak, and at the 

 same time exhibit an alternation of dense and loose layers (e.g. the 

 Birch, in which the layers may be peeled off in thin white sheets) ; 

 sometimes (as in Acer campestre and the Cork-Elm) it forms wing- 

 like projections from the angles of the branches. In a few trees, 

 as the Silver Fir, the primary periderm persists for some years, 

 or, as in the Beech, during the whole life of the tree ; the outer 

 cork-cells split off as the trunk of the tree increases in thickness, 

 while the phellogen, growing and extending in a tangential 

 direction, gives rise to new layers of cork. When, as in most cases, 

 new layers of phellogen arise after a few years in the deeper 

 tissues, leading to the development of corresponding layers of 

 periderm, an external investment of a more or less complicated 

 structure comes to be formed. In consequence of the imperme- 

 ability to water of these secondary layers of periderm, all the 

 tissues lying externally to them become dried up. These dried- 

 up tissues, which may belong to different tissue-systems and 

 include the most various forms of cells, constitute what is known 

 as Bark. When the primary periderm is superficial, the new 

 secondary layers of periderm are only arcs of the circumference, 

 and as their margins are in contact with the periderm which 

 has been previously formed (Fig. 156), a scaly bark is formed, 

 that is, isolated patches of tissue are transformed into bark. 



