384 SUMMARY OF CURRENT RESEARCHES RELATING TO 



dener distinguishes several modes of mechanical thickening, viz. of 

 the cell-walls of the sheath itself; of the walls of the neighbouring 

 cortical cells, as is usually the case in ferns ; of both these ; of the 

 cells of the sheath, and of the layers of cells which bound it on the 

 inside, &c. 



The bundle-sheaths are formed, in the most various ways, from a 

 true cambium as well as from a meristem ; and the thickenings are 

 also either of parenchymatous or of cambial origin. 



Buried Leaf-buds.* — F, Hildebrand describes several cases of 

 adventitious leaf-buds which have become comj)]etely covered over by 

 the surrounding tissue, and concealed by the bark, breaking out, how- 

 ever and develojiing in the following season. They occur in Adinidia 

 polygama, Hhiis glabra, Ptelea trifoliata, Virgilia lutea, Calycanihus 

 fioridus, and Philadeljjhus inodorus. 



Structure of the Wood of Coiiifers.+ — E. Eussow publishes a 

 very detailed account of the structure of the wood, esi^ecially of Picea 

 excelsa and Larix sihirica, with especial reference to the development 

 of the pits and of the membrane of the wood-cells. The general 

 conclusions arrived at are as follows : — 



The vessels and tracheides act as j^umps, which, either by suction 

 or pressure, force the water in the wood from the root to the leaves. 

 The suction is first caused by transpiration, by means of the bilateral 

 pits in the wood ; the positive pressure by the osmotic force of the 

 contents of the paratracheal cells of the medullary rays, and j^aren- 

 chyma of the wood by means of the unilateral j^its. The latter 

 manifests itself especially at the time when negative pressure prevails 

 in the vessels or tracheides, in order to bring about filtration of diffi- 

 cultly diffusible substances in the cells of the medullary rays and of 

 the wood-parenchyma. 



Medullary Rays of Conifers and Dicotyledons.^ — P. Schulz 

 describes the pores in 48 species of conifers occurring in the con- 

 tiguous walls of the tracheides and in the cells of the medullary rays, 

 by which these elements communicate with one another. Those in 

 the medullary rays are always unbordered, while those in the 

 tracheides are sometimes bordered, sometimes not. In some species 

 of Pinus the tracheides which border the medullary rays have I- shaped 

 thickenings, the medullary rays having at the same time large oval 

 pores. They protect the tracheides from the pressure exercised by 

 the turgidity of the medullary cells. The medullary tracheides occur 

 only in the Abietineje, and are of two forms. In Pinus these cells 

 are thickened in a jagged way, and are arranged in several rows ; in 

 Cedrus, Larix, and many sj^ecies of Abies, they are narrowed, have 

 not these jagged thickenings, and are usually arranged in one or two 

 rows in each medullary ray. The bordered-pit-cells are dead and 



* Bot. Centralbl., xiii. (1883) pp. 207-12. 

 t Ibid., pp. 29-40, CO-8, 05-100, lUG-73 (5 pis.). 



I Schulz, P., 'Das Markstralilengewebe u. seine Beziehung zu den leitenden 

 Elementen des Holzes,' 23 pp. (1 pi.) Berlin, 1882. 



