INTRODUCTION 



against the force of the wind ; it conserves the water 

 supply ; under its protecting and fostering care the 

 wreckage of the forest is transformed into vegetable 

 humus, an almost indispensable constituent of a fertile 



soil. 



Probably because it has not yet been made the sub- 

 ject of careful study, the duration of life, even among 

 those shrubs that have been longest in cultivation and 

 are best known, is not very well understood. The 

 short generation, however, implies a relatively short 

 life period. This short generation tends to the multi- 

 plication of species. Seed variations can more easily 

 perpetuate themselves. Bud variations, technically 

 known as sports, in the case of cultivated shrubs, 

 can be readily observed and reproduced within a rea- 

 sonably limited time. So that the limited life of the 

 shrub serves both as an aid and a reward to the ob- 

 servant gardener in multiplying the variants of the 

 most valued species. 



Our northern climate is so favorable to the growth 

 of hardy shrubs that if their value were fully and gen- 

 erally comprehended they would play a much more 

 important part than they now do in lawn and park 

 decoration. Their beauty, unfortunately, is often les- 

 sened, if not wholly destroyed, by careless or igno- 

 rant pruning. Severe pruning tends to enfeeble both 

 shrub and tree, and the removal of large branches usu- 

 ally interferes with the natural and therefore more 

 graceful lines of either. Shrubs will be in their best 

 form and vigor the year through when no pruning is 

 attempted beyond the thinning out of the weaker and 

 overshadowed branches. Moreover, shrubs have a 



XXVlll 



