Lecture XXV. 221 



It is made up of very narrow capillary tubes. The walls of 

 these tubes are lignified, and internal thickenings are to be seen on 

 them. These thickenings in the protoxylem have the form of 

 spiral bands or rings. The two forms often merge into one 

 another, even in the same capillary tube. The length of these 

 tubes is sometimes considerable, but they appear always to remain 

 limited by the length of the original cell which produced them. 

 Their protoplasm does not persist but their cavity is normally 

 filled with water or dilute solutions. Consisting of the empty 

 walls of single cells they are tracheids. Outside the protoxylem is 

 the stouter filament, composed of the primary wood. The primary 

 wood is built up of trie following constituents: (i) Tracheids 

 rather shorter than those in the protoxylem make up the greater 

 part of this tissue. Their walls are moderately thick and lignified, 

 and they are studded over with very minute bordered pits, smaller 

 but essentially similar to those of the Pine. When the walls of 

 the tracheids are very thick the tracheid is called a fibre. (2) 

 Vessels, very long tubes with lignified cell-walls which are 

 closely covered with pits. To the formation of the vessels 

 several, or many, cells contribute : as their side-walls become 

 lignified and pitted their end-walls are absorbed, and so a con- 

 tinuous cavity is formed by the throwing together of a linear 

 series of cells. Between and round about the tracheids, fibres and 

 vessels are found : (3) Short prismatic cells with walls more or less 

 lignified and retaining their cytoplasm and nucleus. These form 

 the wood-parenchyma. There is often a layer of wood-parenchyma- 

 cells round each of the larger vessels. This layer is frequently 

 interrupted where the vessels come into lateral contact with one 

 another or with tracheids or fibres. The location of the spiral 

 and annular tracheids in the protoxylem is doubtless to be 

 associated with the fact that they are developed from the procam- 

 bium, while the stem is still elongating. Those forms of thicken- 

 ing manifestly easily accommodate themselves to this extension. 

 The continuous thickening of the pitted walls occurs consequently 

 only in that part of the wood which is -formed subsequently to the 

 period of great elongation ; and we actually have a record of the 

 change in the rate of elongation of the growing stem in the separa- 

 tion of the turns of the spiral thickenings. The innermost 

 tracheids of the protoxylem have very steep spirals, while in the 

 outer ones the spirals are flat and the turns close together. 



In the veins of the leaf there are conducting tracts quite similar 

 in structure to those of the stem. They are, however, more re- 

 duced in size not only by the diminution in the numbers of 



