CONIFERAE : THE SCOTS PINE 



305 



It is the working out of the details that gives the special characters 

 of the Coniferae. Their habit is easily distinguished from that 

 of the broad-leaved Dicotyledons, and still more easily from the 

 Palms and other large Monocotyledons. A feature which has its 

 bearing upon the habit and 

 spread of the family is the 

 rarity of vegetative propaga- 

 tion. This is in marked con- 

 trast to the Angiosperrris. In 

 Nature it hardly ever occurs, 

 and the forester finds it im- 

 practicable. Virtually all in- 

 dividuals are raised from seed. 

 There is no need to describe 

 the structure of the vegetative 

 organs in detail, since they 

 correspond in essentials to 

 what has been seen in Angio- 

 sperms. It must suffice to 

 note certain features of com- 

 parative importance. The 

 stem is constructed on the 

 same plan as that of the woody 

 Dicotyledons, with indefinite 

 secondary thickening of the 

 vascular ring originating from 

 a cambium (compare Fig. 34, 

 p. 48). It results in the Scots 

 Pine in a woody trunk marked 

 by annual rings and medullary 

 rays, while externally are 

 phloem and a scaly fissured 

 bark (see Fig. 39, p. 5 3) . Resin- 

 passages permeate all the 

 tissues irregularly. They are 

 specialised intercellular spaces, lined by an epithelium, which deposits 

 the sticky resin in the passages. It exudes from them under pressure 

 of the surrounding tissues whenever the plant is broken or cut. The 

 most notable structural feature is that the wood is composed entirely 

 of tracheides, each developed after tangential division from a single 

 cambial cell (Fig. 247). They are arranged, as the cambium is, -in 



FIG. 247. 



Tracheides of Pine, seen in radial section, a-e, are 

 successive tracheides of one radial row. c6= cam- 

 bium. t= young pit of cambium, t', t"= older pits, 

 si = pits of larger area facing the oblong cells of the 

 medullary ray. ( x 550.) (After Sachs.) 



