Botany and Zoology. 503 



2. Recent Advances in Vegetable Histology. — Whether a new 

 classification of the tissues can be properly regarded as an ad- 

 vance, may be open to question, but the following, applied to the 

 secondary structure of wood, appears to have some advantages 

 for comparative histology. It appears in a study of certain orders 

 by Hitzemann, given in abstract in Bot. Centralbl. 1887, vol. 

 xxxi, p. 91. 



A. Elements containing starch. (Parenchymatous system.) 

 I. Parenchyma arranged radially. (Medullary rays.) 



1. Cells chiefly perpendicular to the axis of the stem, generally 

 forming plates of more than one layer of cells. 



2. Cells parallel to the axis, almost always forming plates of a 

 single layer of cells. 



II. Parenchyma arranged tangentially, connecting the medul- 

 lary rays. 



3. Short cells, generally in groups of more than one row. Wood- 

 parenchyma. 



4. Elongated cells, in a plate of a single layer. Substitution 

 fibers and fibrous cells, forming a transition to the next group. 



B. Elements containing no starch. 



I. Fiber-system. (Mechanical system.) Area of cross-section 

 of the element is but a small fraction of that of a longitudinal 

 section. 



5. Pits not present, or very minute, or larger with no distinct 

 border. Libriform-fibers. 



6. Pits present and provided with a large border ; no difference in 

 size or structure between the bordered pits on the side and end 

 walls. Tracheid-fibers. 



II. Trachael system. System for conducting water. 



7. Pits of the side and end-walls show no marked difference 

 in structure, but a great difference in size. Tracheitis. 



8. Pits exhibit noticeable differences in structure, complete per- 

 foration of the partition. Ducts. 



It will be observed that this classification is open to the objec- 

 tion which lies against every system thus far proposed, namely : 

 that both morphological and physiological considerations enter 

 into its construction. But for comparative studies like that 

 undertaken by Hitzemann, the classification seems simple and 

 may prove useful. 



Berggren (in 1883) studied the economy of the spirally-thick- 

 ened cells in the roots of some Taxinese. In the leaves of cer- 

 tain species of Sansevieria, Areschoug (Bot. Centralbl. xxxi, 258) 

 finds similar cells with extraordinary thickness, apparently serv- 

 ing as constituents of a true " water-tissue," instead of being, as 

 in many other well-known cases, unequivocal tracheids. 



Prael (Ber. Deutsch. Bot. Ges. 1887, 417) has lately investi- 

 gated the structure of the wood which forms during the repair of 

 young twigs and stems which have been injured. It has long 

 been known that this new wood partakes somewhat of the nature 

 of heart-wood rather than of sap-wood. The results of Prael's 



