358 



SUMMARY OF CURRENT RESEARCHES RELATING TO 



doveloiimcnt. A transverse section through the cambium-region of 

 a conifer when observed in the darkened field proves that before the 

 primary wall (at once manifesting itself by double-refraction) is 

 formed out of cell-substance, an envelope singly refracting, and 

 consequently not consisting of cell-substance, is secreted out of the 

 protoplasm, which envelope remains during the conversion into wood 

 or bast of the cambium-daughter-cells, and thus becomes the central 

 plate of the " middle-layer." 



As to the share to be assigned to the wall-layers in the formation 

 of the pore-canals and the closure of the pores, Dr. Dippel con- 

 firms Hartii^'s view that it is the innermost layer of the cell-wall 

 which is transformed into pore-canals, and that the closure is formed 

 by two adjoining cells here brought together (fig. 96). The fact, as 



Fig. 97. 



represented in the fig., is admitted by Strasburger, but another 

 interpretation is given to it. According to him the inner layer 

 represents a later difierentiation, which arises in consequence of 

 contact with the cell-contents ; and the more strongly refracting 

 layer, which only apparently extends from the inner plane uninter- 

 ruptedly into the pore-canal, represents just such a differentiation of 

 the secondary thickening in the parts adjoining the pore-canal, whilst 

 the closed end of the pore is formed out of the primary walls. 



Since the collective layers of the cell-wall possess the form of an 

 ellipsoid of elasticity, which, in the transverse and longitudinal 

 sections appears as a section of an ellipse, in which the smallest 

 axis lies radially or perpendicularly to the stratification, and the 

 greater axis parallel to the stratification (the transverse section 

 yielding the least, and the longitudinal section the greatest axis), 

 observation with polarized light must afford the most trustworthy 

 elucidation of the course of the stratification. If the course of the 

 inner layer is as Hartig maintains, then the sections of the ellipsoids 

 must change in the direction as represented in fig. 97. If, on the 

 contrary, the real structure is in accordance with the view of 

 Strasburger, then such a change cannot take place, and the sections 

 of the ellipsoids will be represented in fig. 98. 



In proof that the former view is correct, the author adduces the 



