CHAP, ii The Differentiation of the Tissues 201 



described areas of relative importance quite different from 

 Hanstein's. It ignored the plerome as a definite structure, 

 and raised the pith to an altogether fictitious value when 

 its subsequent fate is taken into account. 



The structure of the apex of the stem in the Gymnosperms 

 was investigated in 1890 by Douliot, who adopted the view 

 that they possess an apical cell, like most Pteridophytes. 

 A memoir appeared in 1891 from the pen of Ludwig Koch, 

 who differed from Douliot entirely on this point. He 

 declined to support the view of histogenetic layers, finding 

 that even the dermatogen is not constantly distinct in 

 Gymnosperms, the outermost apical layer undergoing peri- 

 clinal divisions and thus contributing to cortex as well as 

 to epidermis. Nor did he find a distinct initial group for 

 the adult central cylinder ; on the contrary, he found that 

 from the meristem of the apparent plerome only the pith 

 is derived, the bundles springing from the intermediate 

 meristem. 



Koch returned to the subject in 1893, when he contri- 

 buted a memoir dealing with the Angiosperms, which was 

 the most important that had appeared since the theory of 

 Hanstein had been promulgated. He took the line of 

 making a continuous study of a single plant at the successive 

 stages of its development, and obtained new light upon 

 many points of structure. One of the most important of 

 these was the discovery that the histogenetic layers vary 

 in depth at different stages of growth. Taking a Lilac 

 stem, he saw that a single layer of periblem is present 

 during the formation of an internode ; but that for a node 

 with its pair of leaves this is insufficient. At such a time 

 of formation the periblem meristem becomes three or four 

 layers of cells deep, and it is not always easy to see the line 

 of demarcation from the plerome. These two conditions 

 continually alternate as the growth of the stem proceeds, 

 showing that the apparent periblem layers are not per- 



