Apr. 1P04.] BOTANICAL SOCIETY OF EDINBURGH 457 



than on the under side, often very considerably so, with the 

 result that the Lenticel bearing phellogen is stretched over 

 a wider area on the upper surface than on the under, and 

 the number of primary Lenticels per square centimeter 

 thereby reduced. 



It may, however, be observed that in nearly every case 

 cited with respect to the effect of reduction of pressure on 

 Lenticel formation, an excess of moisture may also be 

 assumed. As regards stomata, the air space on which the 

 Lenticel initials abut is frequently filled with saturated air. 



When a Lenticel arises beneath another tissue, such as 

 the base of a leaf, or tlie cortex of a lateral root, the presence 

 of that tissue above the dividing cells increases the amount 

 of available moisture, and to that extent facilitates division. 

 We may then conclude that at least one partial factor in 

 the formation of lenticellar organs is the reduction of pres- 

 sure above their initial cells, while another in all probability 

 is to be found in the presence of moisture. 



For the history and literature of this subject, Devaux's 

 paper in the " Annales des Sciences Naturelles Botanique"^ 

 should be consulted. 



^ M. H. Devaux. "IJechorches sur les Lenticelles."' "Annales des 

 Sciences Naturelles Botanique,'" ser. 8, t. xii. pp. 1-240, pi. 1-6. 1900. 



DESCRIPTIOX OF PLATE. 



^4. Primary cortex of lateral root. 



B. Primary cortex of parent root. 



C. Pericyclic phellogen of parent root. 



D. Pericyclic phellogen of lateral root. 

 F. Secondary cortex of parent root. 

 L. Lenticel. 



Fig. 1. Longitudinal section of a root of Alder (Alims fjlidinosa) 

 passing through the base of a young lateral rootlet, and 

 showing two Lenticels placed vertically one above and one 

 below the rootlet. The primary cortex of the parent root is 

 already torn and almost cast off, that of the lateral rootlet is 

 still adhering. 



Fig. 2. Transverse section through a similar root, showing two Lenticels 

 lying one to right and one to left of the rootlet. 

 In both figures the young Lenticels clearly lie beneath the cortex 

 of the lateral rootlet. 



