Hazet-wood with Reference to Condtutivity of Water. 55 7 
only. The tracheides are, on the average, narrower than the vessels, but 
their walls are similar, so that in transverse section they cannot be distin¬ 
guished from small vessels with absolute certainty. 
From the fibres both vessels and tracheides are distinguishable in trans¬ 
verse section by the character of their walls. Those of the fibres are 
relatively thick and show few obvious pits, while those of the water-con¬ 
ducting elements are thinner and provided with bordered pits; the latter also 
stain more deeply with safranin. In longitudinal section the vessels are 
seen to have walls studded with numerous small bordered pits, and their 
members communicate by perforations in the transverse walls. These walls 
lie obliquely in the radial plane, the perforations taking the form of a single 
row of large open pits in each. The tracheides are without open pits ; they 
have pointed ends, and their length is thirty to forty times their diameter; 
on the average they are shorter than the vessel members. The fibres have 
narrow tapering ends and a few small oblique pits ; their length on the 
average also exceeds that of the tracheides, especially in the lower part of 
•the stem, where the largest reach a length of 1 mm. and a width of 30 /ot. 
The storage tissue in the wood consists of cells with lignified and pitted 
walls and protoplasmic contents ; they occur in scattered vertical rows, and 
in bands crossing tangentially from one medullary ray to the next; the 
individual cells are elongated more or less in the vertical direction. The 
cells composing the medullary rays are similar in structure, but somewhat 
elongated radially. The rays consist of single, or sometimes double, radial 
rows of cells, and vary in height from one to fifty or more cells, as seen in 
tangential section. All the water-conducting elements are in contact with 
living cells. 
From the point of view of water-conducting efficiency special attention 
must be given to the proportion of the area of the wood in transverse 
section which is occupied by vessels and tracheides; but conductivity may 
be influenced also by the proportion of vessels to tracheides, and by variation 
in the average length of the whole vessels. Since specific conductivity is 
measured in terms of the amount of water passed through at a given 
pressure in a given time, it must be affected by resistance. Vessels will offer 
less resistance than tracheides, in which the water has to pass more often 
through the pit membranes in the end walls; and for the same reason long 
vessels will offer less resistance than shorter ones of the same diameter. 
Thus the elements in question must be compared both in their transverse 
and in their longitudinal characters. 
Method. In order to put into a definite form these general ideas of the 
structure of the wood, and its variation in different parts of the stem, I have 
collected a number of data relating to the size and distribution of the water¬ 
conducting elements. Each shoot was examined in the following way. The 
internodes having been numbered from the base upwards (Fig. 1), transverse 
O o 3 
