ted leaves, which below the epidermis of the upper side ' 

 de-wlop one or more rows of palisade cells, and'helow these, 

 several layers of spongy parenchyma. If the differentia- 

 (47) tion of the mesophyll is arrested, a homogenous leSf tissue 

 is produced, composed throughout of more or less round 

 elements resembling those of the tjrpical spongy parenchsmia, 

 or in which the cells have developed into palisade cells, 

 but in a lesser number of layers than under normal condi- 

 tions. This arrestement of differentiation is recognizable 

 in land plants cultivated under water, in etiolated leaves, 

 in shade leaves, under cultivation, in too great drought or 

 in places saturated vath vapor, with the exclusion of car- 

 bon dioxide, after infection with animal or vegetable par- 

 asites, under the influence of Alpine and northerly cli- 

 mates and, as it seems, also under the- action of other 

 factors, 1 In all these cases, the palisade tissue disap- 

 pears partially, usually, however, entirely, (compare figs. 

 5 and 11) , and a homogenous mesophyll is produced. By the 

 abnormal life conditions named here, the same arrestment 

 formations, so far as leaf structure is concerned, may be 

 obtained in very many different plants and plants of a var- 

 ied nature. Still it is certain, that in plants of char- 

 acteristic life habits, the normal development of many tis- 

 sues presupposes also the fulfillment of special conditions, 

 Thus, for instance, according to J. Schmidt, loc. cit., 

 the normal leaf structure is attained only by providing the 



1, The abundant literature on this question makes more 

 necessary than ever, a limitation to a few examples. Dufour, 

 Infl. de la lumiere s, 1, feuilles. Ann. So. Nat. Bot. vll, 

 series, 1887, T. V. p. 311. Stahl, loc. cit. Yesque and 

 Viet, Infl. du milieu s. 1. vegetaux. Ann. Sc. Wat. Bot. 



VI, serie 1881, T. ZII, p. 17g. Lotheliar, Infl. d. 1' etat 

 hygrometrique et ae 1» eclairement s, 1, tiges et 1. feuiiies 

 d, pi. a piquants. These Lille, 1893j Rech. s, 1. pi. a 

 piquants. Rev, gen, Bot.. 1895, T. V. p. 480. Mer, Rech. 



s. 1, causes de la struct, d, feuilles. Bull. Soc. Bot. 

 France, 1883, T. TDL, p. 110. Schmidt, J. Cm ydre faktorers 

 inflydelse paa Lavbladets anat. bygning hos en af^vore 

 strandpl. Boot, lidske., 1899, Bd. XXII, p. 145. Costantin, 

 Etudes, s. 1. feuilles d. pi. aquatiques. Ann. Sc. Hat.^o-c. 



VII, serie, 1886, T. III» p. 94. Schenck, Ueb. Strukturand- 

 erung submers veget. Landpflanzen. Ber. d. B. Bot, Ges. , 

 1884; Bd. II, p. 481. Keller Biol. Studien I; Anpassungs- 

 fahigkeit phanerog. LandiDfl. an. d. Leben im Wasser. Bioi. 

 Centrabl., 1897, Bd. XVII, p. 99. Duchartre. Ml. ie la. 

 secheresse s. 1. vegetation et la struct. del'Ignamede 

 Chine (Dioscorea Batatas). B^ill. Soc. Bot. France, 1885 



T. XXXII, p. 156. Theodorasco, Infl. de iTacide carbonique 

 s, la forme et la struct, d. pi. Rev. gen.de Bot. 1899, 

 T. XI,. p. 445. Molliard , Rech. s. 1. cecxdaes florales^ 

 Ann. Sc. Nat.- Bot.- 8 me ser^.e, 1895 T. I, p. ^7 . Hartman 

 Anat.. Vergleichun^ d. Hexenbesen d.. Weisstamve m. i- J^^^' 

 Sprossen ders. Dissertation Freiburg i- S^' '/^;^' t'.®'- Mitt 



Einfl. d. alp. Standortes auf d. ^^^f^J^-^^'^f ..^'-.^^^^^ien om* 

 Katurf. Ges. Bern. 1889 .. Borgesen. Bidrag "uil Kundskaben om 



arkiske PI. Bladbygning. Bot. Tidskr. 189o Bd. XIX p. ^l^- 

 Bonnier, Infl. de la lum. elect, continue s. la forme et 

 la struct, d. pi. Rev. gen. Bot., 1895, T. VII, p. 241. 



