218 SCIENCE PROGRESS. 



that hence conclusions drawn from observation of a few 

 sections could not be final, marks a great advance on 

 previous work. Koch claims to have proved (22), in Syringa 

 and Berberis, that the single layer of cells immediately 

 beneath the dermatogen, i.e., the periblem of earlier ob- 

 servers, divides periclinally, during a period of leaf forma- 

 tion, across the actual apex of the shoot, thus giving rise to 

 three or four superposed layers of cells. It is clear that, 

 if this is the case, all but the uppermost of these layers 

 must become part of the plerome when the apex passes 

 back to the state of possessing a single layered periblem. 

 But though our author has convinced himself that this 

 actually happens, his figures are not decisive. Most of the 

 periclinal divisions which he shows in the periblem of the 

 Lilac (Taf. xvi.) are clearly in connexion with the forma- 

 tion of the leaf rudiments. In no case are such divisions 

 shown across the actual apex. In fig. vi. periclinal walls are 

 drawn in two periblem cells removed by one cell from 

 the cell-group obviously concerned in the formation of a 

 leaf rudiment, but these walls are also removed by one 

 or two cells from the centre of the flat growing point, 

 and considering how much this free surface is encroached 

 upon by the developing- leaves {cf. fig. vii.) it is not 

 at all clear that the periclinal wall in question is not 

 precociously formed in a cell which will later be involved in 

 the base of the leaf. Yet this single periclinal wall is 

 really the sole evidence obtainable from his figures of the 

 truth of Koch's view. Nevertheless the thorough method 

 of investigation inaugurated by Koch must sooner or 

 later settle the point. For the present we must admit 

 that though Hanstein's case is made out for a certain small 

 number of plants, the great majority of cases which have 

 been investigated must remain doubtful. Van Tiegfhem 

 (10, p. 776) does not definitely commit himself, though he 

 implies the suggestion that Hanstein's three initial layers 

 are universal in Phanerogams, though often not distinguish- 

 able owing to " enchevetrement" of the layers. But his 

 pupil Douliot (23) concluded that there was a single apical 

 cell in all Gymnosperms, and a plero- periblem in most 



