DEVELOPMENT OF PLANTS 349 



ture of the wood explains the ease with which the timber is 

 worked and the common name of "soft wood" that is apphed 

 to it. The leaves are xerophytic in character and usually rather 

 small and often needle-like. The strongly thickened epidermis 

 is strengthened with strands of stereome that give them a leath- 

 ery character (Fig. 250, C). The stomata are sunken and the 

 centrally-placed vascular bundles communicate with the rather 

 irregular chlorenchyma cells by means of tracheids, and thin- 

 walled parenchyma cells instead of small veins as in the higher 

 plants. The leaves in the majority of the genera remain on the 

 branches for several years, and for this reason the members of 

 the Pinales are commonly called "evergreens". 



These plants generally flourish in localities where there is an 

 abundance of moisture and at first sight the modifications of the 

 leaves noted above are not in harmony with such surroundings, 

 being decidedly characteristic of plants growing in arid regions. 

 The minute tracheids which constitute the conducting system of 

 the Pinales cannot, however, transport the fluids absorbed from 

 the soil as rapidly as the large cells noted among the dicotyledons. 

 Consequently smaller amounts of water are placed at the disposal 

 of the leaves, and were it not for the fact that they are admirably 

 adapted to lessen transpiration these plants would doubtless have 

 disappeared from the earth long ago. The ability of the leaves 

 to diminish the loss of water may also account for the common 

 occurrence of the cone-bearing trees in northern and mountainous 

 districts where the soils are cold and root absorption is therefore 

 lessened and where the vegetation is especially exposed to the 

 drying winds of winter (see page 36). 



Nearly all the Pinales are characterized by the presence of 

 resin passages or ducts in the stems and leaves. Whenever the 

 stems are cut or injured these passages pour out a thick resinous 

 liquid that effectually heals the w^ound. This doubtless explains 

 in part the freedom of these trees from the attack of insects and 

 wood-destroying fungi. It may possibly account for the inabil- 

 ity of many of these trees to sprout from the stump when cut. 

 Hardwood forests will very generally perpetuate themselves by 

 means of sprouts that arise from the stumps, but our evergreen 



