THE MOVEMENTS OF FLUIDS IN STEMS. 211 



becomes less watery, again increasing the pressure on the wood of 

 the stem. We thus see that during the winter the bark is very- 

 tight, while the stem contains the maximum of water, as imbibed 

 water in the walls of the cells, and is also full of reserve matters. 

 A little later the bark cracks and expands, while the cambium 

 begins to be active, hence the pressure is very slight, and the 

 bax'k and phloem easily separable from the xylem at the cambium 

 layer. When in full leaf the cells are empty, the walls full of 

 imbibed water, transpiration being most active at this period, and 

 the rapid current in full flow. Later in the season the reserve 

 matters accumulate in the stem, filling many of the cells ; the 

 walls are full of imbibed water, and the bark tightly contracted 

 on the wood. 



Such, then, is a brief account of the varying conditions of the 

 stem at different periods of the year. 



VII. The felling and seasoning of timber considered physiologi- 

 cally. 



Wood always contains a large quantity of water, hence the 

 absolute necessity of seasoning timber by l-emoving the water and 

 leaving such substances behind as cannot be removed. The 

 object to be secured should be to remove the water as rapidly 

 as possible, and to prevent such changes in the reserve materials, 

 that cannot be removed, as will render them injurious. Warmth 

 and damp, with the absence of a free circulation of air, must be 

 unfavourable to the seasoning of timber. By boiling and steam- 

 ing the wood, most of the reserve materials will be decomposed, 

 and many of them rendered soluble. If these can be removed, 

 then no injury will be done to the wood if now dried ; but if any 

 of the matters are left behind, then moisture and warmth will 

 again produce decomposition. There can be little doubt that 

 natural drying in a free current of air is the best mode of season- 

 ing ; and such wood, if kept dry and exposed to the air, ought to 

 keep well, even although much dried reserve matters, as starch, 

 may be present in the cells. 



When the bark is needed to remain on the stems, the proper 

 time to fell is the winter, when, as already pointed out, the bark 

 is tightest. Care must, however, be exercised, so as not to delay 

 too long, as the cracking of the bark and the consequent relaxa- 

 tion of the tension will begin in February or March, according to 

 the kind of weather, the locality, and latitude in which the plants 



