150 



1JOTANY 



gaseous interchange between the outside air and the gases present in 

 the intercellular spaces of the plant. 



Certain aerial roots such as those of the Orchids and the respiratory roots of 



>\v.-iinp plants possess lenticels the 

 structure of which deviates more or 

 less from the usual type ; these are 

 termed piieumathodcs. 



Absciss Layers. The fall 

 of the leaves of woody plants ( 13 ) 

 is brought about by means of a 

 layer of parenchymatous tissue 

 which is formed at the base of 

 the leaf -stalk shortly before 

 the leaf -fall. The cells of this 

 tissue separate from one another 

 owing to their middle lamellae 

 becoming mucilaginous. With- 

 in this absciss layer all prosen- 

 chymatous elements of the 

 petiole are greatly reduced ; 

 only the tracheal elements are 

 lignified, and these are broken 

 through when the leaf separates. 

 The protection of the leaf-scar 

 is effected in the first place by 

 the lignification and suberisa- 

 tion of the walls of the exposed 

 cells, and later by the formation 

 of a layer of cork continuous 

 with the periderm covering the 

 stem. The ends of the tracheal 

 elements in the scar become 

 occluded by wound -gum 



or 



Fio. 1')!'. Transverse section of the peripheral 

 tissues of the stem of Quercus xessiliflora. 1, 2, 3, 

 Successively formed layers of cork ; pr, primary 

 cortex, modified by subsequent growth ; in- thylosis Or by both ; the ends 

 ternally to ,* 1-ricycle; *c, sclerenchymatous f fcfe gieve . tubes become COm- 

 fibres, from the ruptured ring of sclerenchy- . 



matous fibres of the pericycle ; s, subsequently pressed and lignified. The 

 formed sclereids; s', sclereids, of secondary leaflets of Compound leaves 

 growth ; cr, bast fibres with accompanying crystal 11 r . j e 



ceils; I-, ceils, with aggregate crystals. AII the usually become separated from 



tissue external to the innermost layer of cork the rhachis by absciss layers, 

 is dead and discoloured, and has become trans- Herbaceous plants exhibit as a 

 formed into lark, (x 22'..) , , - \ . . , n ,.,.. 



rule no definite leaf-fall ( 137 ). 



Healing* of Wounds ( 13S ). Lost portions of tissue are in the lower 

 plants frequently regenerated directly, while this only occurs to a 

 very limited extent in more highly organised plants ( 13 ). In the 

 latter regeneration is limited to the periderm, and in a few instances 



