5^4 



SECONDARY CHANGES. 



Prunus Cerasus, Ligustrum vulgare, Syringa persica, Salix fragilis, Rhus typhinum, 

 Fraxinus Ornus, Robinia pseudacacia, and many others ; — or, where the stomata are 

 grouped together, one lenticel is formed under each group, e. g. species of Populus, 

 Juglans regia, and Hedera Regnoriana ; the stoma under wliich the lenticel originates 

 lies over the middle of the latter at a later stage of development, while other neigh- 

 bouring ones may lie in the periphery, e. g. Euonymus europseus, Persica vulgaris, 

 and Cornus sanguinea. 



Even where tlie stomata are less numerous, some of them may remain with- 

 out any share in the formation of lenlicels, as is especially conspicuous in the 



horizontal shoots to be described below, 

 in which the number of lenticels on the 

 upper side is smaller than that on the 

 lower side. In the investigated cases 

 the stomata are here equally numerous 

 on both sides, and on the upper side at 

 least are always more numerous on the 

 same area than the lenticels. 



The formation of lenticels below 

 stomata begins with the growth antl 

 divisions of the parenchymatous cells lying 

 in this position. (Figs. 222 and 223, 

 comp. also Fig. 221, p. 560). The 

 divisions at first take place variously in different directions. Their products, and also 

 no doubt the cells which have not yet divided, grow chiefly in the direction at right 



j: 



Fig. 222. — Transverse section through a young internode of 

 a branch of Betula alba (375). e—e epidermis; n respiratory 

 cavity under a stoma ; c the cuticle, which is separated from e as 

 far as the entrance of the pore by a layer of secretion (removed 

 by alcohol). At .v the first beginnings of the divisions which give 

 rise to the complementary tissue. 



FIG. 223. — Transverse section through a lenticel of Betula alba ; older stage than Fig. 222 (about 280). 

 tf epidermis, j stoma, under the latter the complementary tissue of the lenticel; inside this is its phellogenetic 

 meristem. At the edge of the lenticel the tangential divisions in the hypoderinal parenchyma which give rise to 

 the superficial periderm are beginning on both sides. 



angles to the epidermis ; their angles become rounded, the chlorophyll originally con- 

 tained in them disappears, and the cells acquire the properties of delicate colourless 

 complementary cells. Similar changes now extend further in area and depth from the 

 original point of departure. As this goes on the further dividing walls soon assume 

 a more regular and uniform tangential position, in such a manner that the above- 

 mentioned phellogenetic layer of meristem, which is usually concave towards the in- 

 side, finally appears. The mass of complementary cells lying outside this, in which the 

 divisions soon cease, becomes more and more driven towards the outside, in conse- 



