MISCELLANEOUS PAPERS. 413 



STRUCTURE OF DECIDUOUS TREES. 



Deciduous trees have very fine cells, even microscopic. They range 

 in size from 12 to 25 microns in diameter, and are from 1000 to 2100 

 to the linear inch. Besides the cells proper, or microcells as they 

 really are, all deciduous trees have larger pores or crude sap ducts, 

 which vary greatly in size in the different kinds of wood, being very 

 large in oak, hickory, ash, and elm, and small in maple, poplar, ap- 

 ple, boxwood, and horse-chestnut. 



The ducts, although differing greatly in diameter, are all well 

 within the limits of capillarity. The larger ducts are provided at 

 short intervals with valves to prevent sagging of the sap. 



Among the woods having ducts above the average in size are wil- 

 low, the diameters of whose ducts are from 50 to 110 microns, with 

 much intervening cellular tissue ; hickory, in which the spring ducts 

 are 230 microns across and the ducts of the summer and autumn wood 

 60. In summer grape, in which the spring ducts are very large and 

 oval, they measure 240 microns tangentially and 300 radially. These 

 occupy apparently about one -third the space of the wood. 



In many trees there are two sorts of ducts, the full philosophy of 

 which I have not studied. In the spring formation of wood the ducts 

 are very large, and may be one or several between each two successive 

 radial plates. These ducts are usually well supplied with valves. The 

 valves are often squarely across a duct, but oftener stand through the 

 tube at every conceivable angle and are by no means always a uniform 

 plane, but are frequently a spherical segment. They occasionally 

 run parallel with the walls of the duct and subdivide it into several 

 smaller tubes. Valves contain no pits ; they are thin and require 

 none. When straight across a duct they are usually at nearly regu- 

 lar distances. Frequently two ducts are side by side for a short dis- 

 tance, separated by a septum. Septa are provided with pits for the free 

 transmission of liquids from one duct to another. In the summer 

 and autumn wood the ducts are small, though varying much in size. 

 These are sometimes solitary, sometimes grouped regularly and sep- 

 arated by pitted septa. These tubes contain no valves. Woods hav- 

 ing the two kinds of ducts will be distinguished nominally by calling 

 them heterotracheal, and the two kinds of ducts will be called macro- 

 pores and micropores. 



As examples of heterotracheal wood may be cited oak, hickory, 

 chestnut, and catalpa. In white oak, for example, as in hickory, the 

 larger ducts, or macropores, which occur in the spring growth of the 

 wood, are 200 to 300 microns in diameter ; the smaller ducts, or micro- 

 pores, which are in the solider summer and autumn part of the rings, 

 are 40 to 80 microns. In catalpa, also, the sizes vary much ; the macro- 



