212 TEXTBOOK OF BOTANY 



are visible to tiie naked eye, those in birch and willow are so 

 narrow that they can be seen only with the aid of a micro- 

 'scope ; but there are many more vessels in the wood of a 

 willow than in the harder wood of an oak. The spring wood 

 of the oak is made up almost entirely of vessels, the summer 

 wood mostly of conducting cells and fibers. The vessels 

 of the beech are scattered through both spring and summer 

 wood. Those of the black walnut are in small groups ; so 

 are those of the ash, but here the groups are arranged in 

 straight lines. 



230. Medullary Rays. — These also differ much in differ- 

 ent woods. Since they consist of thin-walled cells, the hard- 

 ness and firmness of the wood are affected by the varying 

 distances between successive rays, as well as by the thickness 

 of each ray. Sometimes, as is often the case in the pine, a ray 

 includes only one layer of cells. In other cases it is two, three, 

 or several cells thick. Oak wood has two distinct kinds of 

 rays, one very thin, the other thick. Rays also differ 

 greatly in the distance to which they extend upward and 

 downward in the trunk. In a board that is cut from a tree 

 lengthwise, in such a way as not to pass through the center 

 of the trunk, the medullary rays are cut more or less nearly 

 straight across. If the board is so cut that it takes in the 

 center of the trunk, the medullary rays are not cut crosswise 

 but are parallel, or nearly so, to the surface of the board. A 

 board cut in this latter way is quarter-sawed, and it is the 

 markings due to the medullary rays that make quarter- 

 sawed oak so beautiful a wood. 



231. Milky Juices. — Some common plants, among them 

 the milkweeds and the spurges, produce a white, sticky juice 

 that runs out when a stem or a leaf is broken. This juice is 

 formed in certain long cells in the cortex of the stem and 

 leaves. The liquid is a mixture of many substances. The 

 milky juices of many plants, including the milkweeds, contain 

 rubber. A good many tropical and subtropical trees and 



