DEPILATION. PERIDERM 



133 



or which predetermine the point of rupture, are but rarely found. 

 The author has noticed that the deciduous unicellular hairs on the 

 upper side of the leaf of Coscinium Blumeanum are noticeably con- 

 stricted towards their lower ends. A thin strip in the wall indicates 

 the point at which the T-shaped unicellular hairs of Banisteria break 

 across. Multicellular hairs are usually cast or disarticulated : that is 





Fir.. 30. 



Occlusion of atrophying glandular hairs on the adaxial leaf-surface of Rosmarinus 

 officinalis. The lateral walls of the stalk-cell become enormously thickened, and the 

 cavity is largely obliterated. When the rest of the hair breaks off, a thick-walled 

 almost solid papilla is left. 



to say, the transverse wall separating the deciduous part of the hair 

 from the persistent basal cell or foot splits across. Those layers of 

 the wall which ultimately become exposed undergo cutinisation before 

 separation takes place. The various modifications of this process of 

 occlusion by means of cutinisation or thickening of the cell-wall, which 

 may be seen in certain Papilionaceae, Proteaceae, Compositae, etc., 

 cannot be discussed here. A similar occlusion not infrequently takes 

 place in connection with the atrophy of glandular hairs ; in this case it 

 probably serves to maintain the continuity of the dermal tissue, and 

 thus not only to prevent the occurrence of any uncontrolled transpira- 

 tion, but also more particularly to hinder the entrance of parasitic 

 Fungi. 



///. PERIDERM. 



When an organ has reached a certain age the epidermis as a rule 

 no longer affords sufficient protection for two reasons. In the first 

 place, this layer is after all a comparatively delicate structure, and is 

 consequently not well fitted for the protection of bulky organs such as 

 the branches of woody plants. Secondly, apart from a few exceptional 

 cases, the epidermis is incapable of keeping pace with the active growth 

 in thickness which most axial organs exhibit, and therefore soon 

 becomes ruptured and torn into shreds. If, therefore, an organ which 

 is growing in thickness is to remain enveloped in a protective covering, 

 the epidermis must be replaced by a dermal tissue which is tougher 



