STEM OF TILIA. 



157 



wall is perforated by a round opening, or closed by bordered pits, 

 determines whether the element in question is to be distinguished 

 as a vessel or a tracheide. The greater proportion of the wood 

 is built up of long-pointed wood -fibres, the walls of which are 

 provided with small cleft-like pits obliquely rising to the left, i.e., 

 following a left-handed spiral. Like vessels and trachei'des, the 

 wood-fibres also are without protoplasmic contents. While, 

 however, vessels and trachei'des serve for the conduction of water, 

 the wood-fibres in fresh wood contain air, and only serve, there- 

 fore, to enhance the mechanical stability of the woody mass. 

 They are to be reckoned rather as a portion of the stereome. 

 The wood-parenchyma cells follow one another in uninterrupted 

 vertical rows ; they are short, separated by simply pitted walls, 

 with protoplasmic contents and usually, also, filled with starch. 

 The medullary rays (Fig. 61) cross the elements of the woody 

 mass, as bands of very varied height. 

 The edges of these medullary rays 

 are composed of shorter or higher 

 cells (tm), and we can see that it is 

 these latter alone which are con- 

 nected with the vessels by means of 

 numerous large pits. Unilamellar 

 medullary rays can be composed 

 exclusively of the higher cells. The 

 medullary-ray cells connected with 

 the vessels by means of pits are 

 marked by poverty in starch. 



In the bast we note, above 

 everything, the very long shining 

 bast - fibres ; we again find the 

 starch - containing, and also the 

 crystallogenous cells of the bast- 

 parenchyma ; then the sieve-tubes, 

 the tangentially-inclined sieve-plates 



sm 



FIG. 61. Fragment from a radial 

 longitudinal section of the wood of 

 Tilia with a small medullary ray. 

 g, vessel ; /, wood-fibre ; tm, medul- 

 lary-ray cells, connected by pits with 

 the water-route ; sm, conducting me- 

 dullary-ray cells (x 240). 



of which are cut up into separate sieve-areas. The narrow com- 

 panion-cells are distinguished by their rich contents. Outside 

 the bast, and if the stem be fairly young, we note the collen- 

 chyma with the white thickened angles, and the cork-cells, 

 which have exactly the same form in longitudinal as in cross- 

 section. 



The tangential longitudinal section through the wood shows 



