THE MOLECULAR FORCES IN PLANTS. 170 



solution begins to appear in the glass tube. The values obtained 1 

 always refer to the longest of the vessels present. In three-year- 

 old shoots of Corylus avellana the length is about 11 cm. ; the 

 vessels in Aristolochia twigs are very long, often as much as 200 

 cm. (six-year-old shoot). 



We now treat in exactly the same way a 10 cm. piece of Alnus 

 twig several years old. When this, after continued exhaustion, is 

 sufficiently injected with the iron solution, we dip the lower end 

 of it into a mixture of 1 part of commercial ammonia and 3 

 parts of water, and exhaust further. By microscopic investiga- 

 tion of longitudinal sections, and by comparing successive trans- 

 verse sections, we determine up to what height the red-brown 

 precipitate thrown down by the ammonia solution reaches in the 

 vessels. At these limits are. also to be seen, especially in 

 tangential sections, the oblique, imperforate, division walls of the 

 vessels. 3 



It is a fact of great importance that the membranes of the 

 elements of the wood are very highly impermeable to air, even under 

 considerable pressure. 4 This fact is of high significance for the 

 theory of the negative pressure of air in wood, and also for the 

 theory of the movement of water in plants, and it gains a very 

 special interest, if we place it side by side with the fact, which 

 we shall establish later, that the same wood substance which is 

 penetrated with such difficulty by air, offers scarcely any resist- 

 ance to the movement of water. 



We can best satisfy ourselves that the wood substance does offer 

 very great resistance to the passage of air by using pieces of 

 wood, a few centimetres long, and about the thickness of a finger, 

 cut out from the youngest annual rings of newly felled trunks 

 of Taxus baccata or Abies pectinata. Such a piece of wood is 

 fixed air-tight in the shorter limb of a bent glass tube by means 

 of rubber tubbing. We then pour mercury into the longer limb, 

 and place the apparatus in a glass cylinder filled with water (see 

 Fig. 63). If we use fresh pieces of wood, or still better fresh 

 pieces of wood which have been lying for some time in water, a 

 pressure of 76 cm. of mercury is often insufficient to force air 

 through. We shall see in another place that the tracheides of the 

 wood do not directly communicate with each other. We shall also 

 see that water can filter with the utmost ease through the closing 

 membranes of the bordered pits of the tracheides, while air can- 

 not traverse them even under high pressure. If we find that by 



