210 THE MOVEMENTS OF FLUIDS IN STEMS. 



a result of the growth in the cambium layer, and the tension of 

 the cells from the water in the walls, great and increasing pressure 

 is exercised on the bark, which, from its unyielding condition in 

 the summer and autumn, renders the cells formed in the late 

 summer smaller in size, and with greatly thickened walls, the 

 mici'oscopic examination of the stem exhibiting a marked contrast 

 in the relative appearance of the early and late formed wood. 

 These differences have been pointed out and fully described by 

 Professor Kraus of Erlangen, and the cause demonstrated by 

 experiment. 



VI. The condition of the stem at different seasons of the year. 



The different conditions of the stem at different seasons of the 

 year are of great importance from an arboricultural point of view. 

 Thus in winter the cells of the stem have their walls full of 

 imbibed water, the cavities of many of them being also filled with 

 reserve matters and water. Hence it is that in winter the 

 pressure of the wood on the bark is very great, and it may be 

 that dining this time a certain amount of consolidation of the 

 newly formed wood is going on. At the same time the cold is 

 acting on the bark, which has now become veiy dry, and is there- 

 fore exercising a great deal of pressure on the wood, the wood and 

 bark being mutually antagonistic, and by their action giving rise, 

 to some extent, to the well-defined line of separation between the 

 annual layers of wood. After a time the pressure of the wood 

 overcomes the resistance of the bark, and gives rise to cracks and 

 fissures in it, or the widening and deepening of cracks already 

 formed. It is to be noticed that such cracks usually enlarge con- 

 siderably in the early part of the year, as in February and March, 

 from the wood pressure. The next stage is due to the improving 

 weather and the increase in temperature and moisture. The 

 bark swells up by imbibing water and the rise in temperature. 

 This causes in its turn a relief of the tension on the wood, the 

 bark being hardly, if at all, elastic. The effect of the swelling 

 and loosening of the bark is to give the cambium free scope for 

 developing. As the growth of the cambium goes on, and new 

 xylem and phloem are formed during the summer and early 

 autumn, the space thus formed by the swelling of the bark has 

 been filled up and the tension again gradually increases. The 

 wood cells now formed are smaller and smaller, and have thicker 

 walls. As the weather becomes colder the bark contracts and 



