170 OHIO EXPERIMENT STATION: BULLETIN 332 



hence when a pest becomes established in the midst of a pure plant- 

 ing, it is afforded exceptional advantages for development and 

 spread. As concrete evidence of the point, attention is called to 

 Plate II, Fig. 1. As a fiirther illustration of the fact, take for in T 

 stance the case of Marietta, the most beautiful city in the State 

 with respect to tree planting, where it is the writer's belief that 

 nine-tenths of the trees are of a single species, the white or Ameri- 

 can elm. Some years ago, one of the elm scale insects, Gossyparia 

 spnria, became established in the city, and at this time by reason 

 of its having had an abundant, available food supply has become 

 widely disseminated. Today the infestation bids fair to assume 

 serious proportions unless diligent measures are taken to curtail it. 



Planting of small units to a single species commendable. While 

 the writer is thoroughly opposed to the plan of devoting large areas 

 to a single species, he is equally positive in his views that the plant- 

 ing of pure stands in smaller areas is desirable. Just where the 

 dividing line should fall is difficult to say. It appeals to him as 

 doubtful policy, however, to group more than a score or two of a 

 given species, but in street planting several blocks or perhaps an 

 entire street may be uniformly planted without involving any great 

 danger. When the trees are in the extended planting of the street 

 line, scourges among them do not spread nearly so rapidly as when 

 they are grouped. 



Some of the advantages of planting in units of the same species, 

 especially in street work, may be enumerated as follows: 



In street planting, formality is to be desired simply because 

 any other scheme can hardly be employed to good effect, and it is 

 rarely possible to secure this or uniformity either when trees of 

 more than one species are used. 



The unit planting scheme makes for uniformity and when the 

 planting is of such a nature it is much more convenient to dispose 

 of any electric or other wire lines which must be present with a 

 minimum amount of injury to the trees. 



The trimming of a uniformly planted street is much simplified, 

 but one of the greatest advantages of all is the bearing it has on 

 the work of spraying. When the trees are of a single species, they 

 are much more likely to admit the use of a single spraying material 

 and all may be treated at a given time. Moreover, when uniform in 

 size, changes in the apparatus are not constantly necessary in order 

 to adapt it to the treatment of trees of varying sizes. If the trees 

 are of mixed sorts, one may require a given treatment; the next 

 quite a different one, and a third no treatment at all. If small and 



