10. 



-o o>- ^Ividin- nntivfil". Snrtio^- I cuts off a dermatoren 

 layer by a pericline near the outer end of eacli half, t.lien 

 a second plerome portion is cut off :^rnn thn rmer end of 

 oacli (d« and pl.^ Fi'> 13) and section II , follov/ed hy 

 each of tlio otliers in turn, cuts off plerono nn^i derraato ''en 

 (Figs. 13 - irO, while tho ultimate marr^inal cell Is last 

 of all to forrn^ first the deraato'^en wall as described ahove, 

 and then still later the plerome wall (pi. w.^ Fi/^. 1^) » TJie 

 dermator;en soon splits "by a pericline into two layers a- 

 round the whole circu:nference of the petiole, and tliese 

 hreak up "by anticlines, "botli longitiidinal' and transverse, 

 to form tlie epidermis and hj-'podermis, each, of v/hicli remains 

 of one cell in thickness even in the mature leaf (ep.hy. 

 Fifrs. 14 - 19). On the line of the median wall, of eac. of 

 the section wall ^j and of the halvin^ anticline of sectior; I, 

 small intercellular spaces soon appear hetween th.e cells 

 of the periblem and tliose of the hypodermis (a.c.,Fi^s. 

 13 - 19). T}iese are the bef^innings of the longitudttnal air 

 canals of t]ie petiole, of v;hich, as can "be seen from th.eir 

 origin, fourteen are ferried altogetlier in leaves v/here tlie 

 segments divide accordin'^ to this type. 



The sin,";le periblem cell formed in each half of sec- 

 tion I , in eacli of th.e otlier sections and in tlie ultimate 

 marginal cell (ph. Fi'-;s. 1?, - I-'')/ soon divides uy a peri- 

 cline into tv.'o cells (Fi^. 15). Of tliosc tlie inner cell di- 



