154 MASS. EXPERIMENT STATION BULLETIN 170. 



in the case of gas leaks in the street. Many cases are known to the writer 

 where large trees have escaped gas poisoning owing to the fact that when 

 the curbing was put in some of the larger roots leading towards the gas 

 main were destroyed; therefore when leakage occurred there were no 

 roots favorably located to absorb the poisonous substances. 



The cutting of roots on vigorous trees is not so serious as cutting those 

 of old trees. In the latter case judgment should be exercised as to root 

 cutting. 



Effects of Light and Shade. 



Most plants are quite susceptible to light and shade. Those which 

 require light are termed photophilic (light friendly), and those which 

 thrive best in shade, photophobic (light shunning). Shade has an unfavor- 

 able effect on plants, causing a spindling growth and rendering them 

 more susceptible to diseases. On the other hand, too much light is detri- 

 mental to certain species. The dense shade from street trees interferes 

 at times with the growth of grass and shrubbery on lawns. Since there 

 are relatively few varieties that are adapted to shade, it often becomes 

 a problem as to what to plant in such locations. However, a glance at 

 any native flora will give a hint of what is best adapted to shady places. 

 Such wild species as clethera, rhododendron, hobblebush, leatherwood, 

 moose and mountain maples, laurel and honeysuckle tolerate shade, and 

 there are some exotic shrubs, such as Ligustrum regelianum, Sy^nphoricarpus 

 vulgaris, Xanthorrhiza apiifolia, etc., and vines like Euonym.us radicans 

 and Vinca minor, that are adapted to shade. 



Notwithstanding the fact that shade is natural to some species, they 

 develop a less spindling growth in light. Shade trees require light; hence 

 for their best development they should be planted far enough apart to 

 prevent interference and spindling growth. The effect of shade on trees 

 when growing thickly together is a dying of the lower branches, inducing 

 growth in height at the expense of spread of the crown and growth in 

 diameter. 



The variation in light intensity differs, as is well known, during the 

 year. Light intensity is also variable in different localities, and there 

 are definite variations that occur in light intensity during the day which 

 are more pronounced at some seasons of the year than at others. The 

 difference in the amount of sunshine peculiar to any region is not depend- 

 ent on latitude but on other conditions. For example, the number of 

 hours of total sunshine occurring durmg the j^ear at Boston, Mass., is 

 2,493; Cleveland, Ohio, 2,075; Chicago, 111., 2,616; Milwaukee, Wis., 

 1,865; Seattle, Wash., 1,973; Elkins, W. Va., 1,737; Phoenix, Ariz., 

 3,742, and New Orleans, La., 2,378. These marked variations in the 

 number of hours of sunshine show that latitude does not necessarily con- 

 stitute an important factor in determining light conditions. The amount 

 of possible sunsliine, according to the United States meterological ob- 

 servatories, varies from 37 to 84 per cent. Variations in light intensity 



