SECT. I 



MORPHOLOOY 



161 



mentary cells are as a rule not suberised, those of the closing bands 

 are corky and lignified. The cork- forming phellogen joins the phel- 

 logen of the lenticels at its margins (Fig. 167). In cases where the 

 cork is more deeply seated in the inner tissue, the lenticels begin their 

 development at a corresponding depth from the layer of phellogen. 

 Lenticels serve to maintain the 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 

 swamp plants possess lenticels the structure of which deviates more or less from the 

 usual type ; these are termed pneumathodes. 



Tyloses. — In the vessels of both the secondary and the primary 

 x^dem of stems and roots sac-like 

 structures more or less completely 

 filling the lumen may be met wnth. 

 These are termed tyloses, and 

 originate by the closing membranes 

 of pits between the vessel and 

 adjoining parenchymatous cells 

 bulging into the former, and the 

 intrusive cell continuing to grow. 

 The formation of tyloses may be 

 brought about by wounding the 

 organ ; they close the vessel ex- 

 posed by the wound and hinder 

 its drying up. Such tyloses are 

 spoken of as traumatic. But 

 tyloses also occur in uninjured 

 tissues, and frequently in the heart- 

 wood (Fig. 169) where they close 

 up the functionless vessels. They 

 also occur in still functional vessels, 

 but then only tend to partially 

 fill the lumen and serve for the storage of reserve-substances such 

 as starch (^^°). 



Absciss Layers. — The fall of the leaves of woody plants (^^^) 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 lamell?e becoming mucilaginous. Within this absciss layer all 

 prosenchymatous elements of the petiole are greatly reduced ; only 

 the tracheal elements are lignified, and these are broken through when 

 the leaf separates. The separation of the leaf results from the round- 

 ing off of the cells of the absciss layer, and in some cases this is 

 associated with an elongation of the cells of the outer layers. The 

 protection of the leaf-scar is el.ected in the first place by the lignifica- 



M 



Fio. 1(30. — Transverse section of a vessel from 

 the heart- wood of Eobinia Pseudacaoia, closed 

 by tyloses ; at a, a, is shown tlie connection 

 between the tyloses and the cells from which 

 they have been formed, (x 300.) 



