MISCELLANEOUS PAPERS. 421 



CIRCULATION OF THE SAP. 



Sap passes from cell to cell through minute oval or circular win- 

 dows, quite regularly placed, in rectilinear rows when in a duct be- 

 side vertical cells ; transverse rows when beside the horizontal cells 

 of the radial plates ; scattered or in somewhat spiral or diagonal rows 

 when beside a similar duct, and yet differing in character in the dif- 

 ferent families of trees. 



These windows are minute pits with a thin transparent or translu- 

 cent film of cellulose stretched across. Greatly magnified, a section 

 of duct looks like a section of well-point of a driven well, or like a 

 section of perforated boiler-iron with soap-bubble films across the 

 perforations. Each cell has from four to six rows of these pits com- 

 municating with the adjacent cells, being one row to each adjoining 

 cell. 



The number of these pits to the inch in a straight line is about the 

 same as of cells of the deciduous trees, say 2000. The diameter of a 

 pit when circular is about four microns; when oblong is about six 

 microns the longer way, four the shorter way. With the marginal ring, 

 or frame of the window, the measurement across a pit is about nine to 

 twelve microns. The ducts often have 50 to 100 or more vertical rows 

 of pits communicating with adjoining cells and ducts. The size, 

 shape and distance apart of the pits in the ducts of any tree is pre- 

 cisely the same as in the vertical cells, but usually much larger than 

 in the ray cells. 



Sap flows upward in the ducts of deciduous trees and the cells of 

 coniferous trees, mainly in the sap-wood ; downward between the bark 

 and wood and through the inner layers of bark ; inward in the radial 

 plates; and permeates all through the cells of deciduous trees, except 

 some of the heart cells, which are filled with wood fiber and various 

 organic substances. 



Whatever the diameter of the cell, or whatever its length, water 

 will percolate or creep to the extreme end, even against gravitation, 

 by simple capillarity, aided by osmotic pressure. The bud or leaf of 

 the plant or tree has no such sucking power as has been ascribed to 

 it. It has no sucking power at all. The food comes to the bud or 

 leaf by processes over which it has no control and cannot aid in any 

 way except by making use of the materials that come to it as food. 

 And that it does and does it well. 



OSMOSIS. 



From cell to cell, water, carrying with it its mineral contents, al- 

 ways of a crystalline nature, leaving bacteria, fungi spores, and albu- 

 minoids and colloids behind, passes by a molecular process called 

 osmose, always from a cell containing water of any gravity to a cell 



