Chap, ii The Differentiation of the Tissues 197 



but not showing differentiation into layers. This is only 

 found in certain Cucurbitaceae and Leguminosae. 



Treub published the results of some researches on the 

 same subject in 1876. He agreed in many points with 

 Janczewski, but found that the second type does not prevail 

 so widely as the latter thought. He accordingly divided 

 the group into two, the latter of them having a well-defined 

 plerome, covered by a group of meristem two layers thick, 

 from which the root-cap, dermatogen, and periblem take 

 their origin. 



In 1875 Famintzin affirmed that the different adult 

 tissues of the plant are universally derived from definite 

 histogenetic layers ; he held the latter to be separated 

 even in the embryo and to develop independently like the 

 germinal layers of the animal body. Fundamentally his 

 histogens were the same as Hanstein's. 



Researches generally confirmatory of those of Janczewski 

 were conducted by Eriksson in 1878, Flahault in 1878, and 

 Olivier in 1881. 



These views obtained general acceptance for several years. 

 Van Tieghem and Douliot, who brought forward a further 

 development of Hanstein's fundamental idea a few years 

 later, adhered to the theory of the histogenetic layers in 

 the root as late as 1889. A year later Douliot found in 

 the stems of Dicotyledons in general the three layers of 

 Hanstein, but in those of Monocotyledons only a dermatogen 

 distinct from a common initial group giving rise to both 

 periblem and plerome. 



The second and third types of Janczewski, said to be 

 characteristic of the roots of Monocotyledons and Dicoty- 

 ledons respectively, were interpreted differently by Van 

 Tieghem in 1891. He held that the outer layer, which he 

 called the epiblema, is the outer layer of the periblem 

 in the former, and that no dermatogen exists there ; but 

 that in Dicotyledons it is the result of the tangential 



