90 GENERAL BOTANY 



layers of summer and spring wood. The annual rings determine 

 the grain of wood, which is said to be coarse if the annual 

 rings are wide, and fine if the rings are comparatively narrow. 

 Texture, which refers to the coarseness or fineness of the wood 

 elements, also enters into the designation of grain as coarse or 

 fine. Again, the grain is straight if the woody elements of the 

 annual ring run straight up and down (Fig. 48, d), or it is 

 wavy or curly if these elements take an undulating course 

 (Fig. 48, e and/). In this latter case the grain may be desig- 

 nated as curly, in curly birch, or as bird's-eye, in bird's-eye maple 

 (Fig. 49). Silver grain of wood is due to the wood rays, espe- 

 cially when the latter are cut radially, as is the case in quarter- 

 sawed oak (Fig. 50). The grain, whether due to annual rings 

 or to wood rays, is also greatly modified in appearance in fin- 

 ished woods by different modes of cutting or sawing the original 

 log or piece from which the finished product is prepared. The 

 student should in all cases supplement this brief text descrip- 

 tion with critical observations of finished woods displayed in 

 the furniture and the finishings of the laboratory or the home. 



MICROSCOPIC STRUCTURE 



In the above discussion of the gross structure of woody 

 stems the main layers have been outlined under the terms. bark, 

 cambium, wood, and pith. It remains, therefore, to discuss these 

 layers somewhat more in detail as they are seen in thin section 

 under a compound microscope. In such sections it may be 

 observed that each of the above gross subdivisions of a stem is 

 composed of one or more groups of highly differentiated cells 

 called tissues. Each tissue is a group of cells, similar in structure 

 and in function, which has differentiated from the products of 

 the meristem cells formed in the bud. 



The bark (Fig. 52) is a complex layer made up of dead tissue 

 elements, which serve for protection and mechanical support, and 

 of living, active cells, which are concerned with the life processes 

 of the stem. It is composed of the following tissues : epidermis, 

 cork, cortex, and phloem. 



