136 



PHYSIOLOGICAL BOTANY. 



lines which appear in a cross-section, passing like radii from the 

 pith to the bark, intersecting the wood and dividing it into 

 wedge-shaped bundles or sectors. They consist of firm plates 

 of parenchyma (muriform tissue, the cell resembling brick-work) 

 belonging to the same system with the pith. 



417. The medullary rays are no less frequent in the outer layer of wood than in the 

 h ner. Hence, their number must increase yearly, and a new set commence with each 

 5' (xcssive layer, extending with those already formed through the subsequent laj'ers to 

 tae bark, as shown in the diagram (509). In a radial section (511. 512) the medullary rays 

 are more conspicuous as shining plates of a satin-like texture, called the silver-grain, 

 quite showy in Oak, Maple. A tangential section shows their ends in the form of thin 

 ellipses. They serve as bonds to combine into one firm body the successive wood-layers, 

 and as channels of communication to and from the bark and heart-wood. They also gen- 

 erate, at their outer extremities, the adventitious buds. 



418. The cambium layer. Between the liber and the 

 wood there is formed in the Spring, at the time of the opening 

 of the buds, a mucilaginous, half- 

 organized layer of matter. Its 

 presence loosens the bark, and 

 renders it easily peeled from the 

 wood. The cambium is a sap 

 solution of the starchy deposits 

 of the preceding year, now being 

 rapidly organized into cells. 



419. This is the generative layer, whence 

 spring all the growths of the ligneous sys- 

 tem. From this, during each growing season, 

 two layers are developed, one of liber and one 

 of wood, both at first a cellular mass, but the 

 cells with wonderful precision transforming, 

 some into the slender bast-cells of the liber, 

 some into the dotted ducts and fusiform cells 

 of the wood, some into the muriform tissue of 512, Wood of Maple 

 the medullary rays. Through these latter 

 the quickening influence of the cambium pervades both wood and bark. 



420. Unlimited growth is therefore a characteristic of the exo- 

 genous stem ; for the yearly increments are added to the out- 

 side of the wood, and the bark is capable of expansion 15y lateral 

 growth to any extent. 



ttevtew. What does the "Ligneous System" include? 404. Name the four grand 

 divisions of plants. 405. Describe a cross-section of Exogen. 40H. The pith its compo- 

 sition and contents. 407. The medullary sheath its composition, connections, office. 

 408. Of what does the wood consist? Ho'w much grew the first year? Each successive 

 year ? The rule ? 409. Exceptions ? 410. What is the alburnum ? How does it become 

 duramen ? Which is valuable as timber? What has perished in a hollow tree? 411. 

 Name the three layers of the bark. 412. Tissues of liber? Why so tough? 413. Tissue 



, mecbllary toys; &, ducts; 

 c, wood-cells. 



