REVIEWS. 405 



gated by Russow, Sariio, and others, the details of a con- 

 crete and critical example are offered to us. 



Shortly put, the radial cell-walls of the cambium are 

 thicker than the tangential ones, and become relatively 

 thinner (by stretching and loss of water ?) as they pass out 

 of the younger zones. In the young wood we find the 

 faintly-continued origin of the well-known " bordered pits " 

 (Tupfel) on the radial walls only, and in a primitive con- 

 dition. On the thin plate in the centre arises, as a circular 

 cushion, the '* torus ; " this is a thickening of the primary 

 wall at this place. 



The cell protoplasm now begins to bring *' microsomes " 

 to the surface, and the torus becomes thickened. Soon 

 after the ring is formed, which, by increase in breadth, 

 arises obliquely from the edges of the " primordial-tupfel,-' 

 and changes it into the finished Hoftilpfel — the finished 

 "bordered pit." 



This ring does not become formed on all the primitive 

 pits, however ; many of them disappear later. In other 

 cases a faint circular line around two complete " bordered 

 pits " indicates the remains of a primitive pit on which 

 two rings have formed. 



The thickening of the wall of such wood-cells begins with 

 the origin of the rings above described, and proceeds with 

 the growth of the latter; at the same time the primary cell- 

 wall, on which the thickening layers are deposited, becomes 

 chemically changed. The ring consists of continuations of 

 the thickening lamellae, each passing beyond the edge of 

 the former one ; and the thin, altered membrane passing 

 over all and lining the inside of the pit, must be 

 looked upon as arising by alteration of the edges and sur- 

 faces of lamellae exposed to the continued action of their 

 environment. 



Space does not admit of our discussing other observations 

 on this difficult histological subject ; enough that Stras- 

 burger gives a fair resume of earlier and other views. The 

 secondary thickening layers, described above as deposited 

 successively on the primary wall, often exhibit spiral stria- 

 tions; these may be broad or narrow, and commonly cross 

 the long axis at about 45°. Strasburger fails to confirm 

 Naegeli's observation that these spirals may cross one another 

 in the same layer ; they do not appear on the ring of the 

 bordered pit, but they may influence the direction of the 

 latter. The striae may be so close that they can only be 

 seen in H2S0^, &c. We must pass over the discussion re- 

 lating to the earlier views as to the nature of these stria? 



