140 A PINE. 



the wing and time the fall of the seed alone. What 

 would be the effect in a wind? 



k. Make a drawing illustrating the carpel, and the ovule 

 with its various parts as outlined above. 



ANNOTATIONS. 



The pine raises a strong tall stem above the ground, 

 thus better exposing its leaves to the light. This stem 

 also serves better to expose the flowers so that pollination 

 may be effected and that ripened seeds may be distributed. 

 This stem habit is made possible through the extensive 

 development of mechanical and supporting tissues. 



The stem by means of its terminal buds may continue 

 its growth in length, and also may continue its growth in 

 diameter by means of the growth cylinder the cambium 

 layer that lies between the xylem and phloem. The 

 difference in the size and form of the cells added to the 

 xylem at different times in the growing season gives rise 

 to markings or annual growth rings that indicate the 

 age of the stem. 



In the details of its structure the stem is somewhat 

 complex. In its central part is a pith region composed 

 of ordinary parenchyma cells. In the innermost region 

 of the xylem are a few greatly elongated, tubular cells, 

 trachea, which are not found in the secondary wood of 

 pines, but are of especial interest since they appear 

 abundantly in the wood of the next and highest group, the 

 Angiosperms. In the Gymnosperms the bulk of the wood 

 is almost exclusively made up of tracheids. In the de- 

 velopment of the vascular tissue the cells become greatly 

 elongated, and in tracheae most of the end walls of the cells 



