REPRODUCTION AND DISPERSAL 891 



the initial vegetative period {i.e. the period from germination to anthesis) 

 and the length of the reproductive period are subject to wide modifica- 

 tion through the operation of external factors, and it has been found 

 possible also to extend the initial vegetative period indefinitely by the 

 inhibition of reproduction. These experiments shed much light upon the 

 phenomena cited in the preceding pages. Many interesting facts con- 

 cerning reproductive periods have long been recognized, because of their 

 important practical bearing. For example, crops like peas, tomatoes, and 

 sweet corn may mature some days or even some weeks sooner on dry, 

 well-lighted slopes than in rich, moist lowlands, so that the profit from 

 the former is often much the greater; but in crops where the vegetative 

 organs are marketed, the rich, moist habitat often is preferable, because 

 of the greater luxuriance of the foUage. Comparable phenomena are 

 exhibited by trees, Pinus silvestris maturing fruit in fifteen years if stand- 

 ing alone in dry soil, but requiring thirty to forty years in a grove ; simi- 

 lar differences are seen in the beech and in many other trees. 



Early reproduction, which often is of great practical benefit, frequently 

 is brought about by various mechanical means. Picea excelsa, which 

 commonly flowers in thirty to forty years, may be induced to flower in 

 four to ten years by transplanting, especially if the roots are injured. 

 Orchard trees often fruit much better, if some of the roots are removed. 

 Girdling sometimes induces flowering in apple trees that otherwise 

 exhibit only vegetative activity. Shoots of a young tree grafted on 

 an old tree bloom much sooner than those that are left on the young tree.' 

 Cuttings bloom long before seedlings, a matter of the highest economic 

 importance. Of much interest is the fact that a cutting from an old plant 

 blooms much sooner than one from a young plant, though the cuttings 

 may be of equal size and similar aspect. This phenomenon is most 

 strikingly displayed in leaf cuttings (as in Begonia or Achimenes) , in 

 which the young shoot flowers almost at once if the leaf is taken from 

 a flowering plant, but only after a long time if taken from a young plant. 

 This phenomenon has been explained by postulating the accumulation 

 of flower-forming substances in plants approaching maturity, but this 

 assumption needs explanation as much as do the facts which it attempts 

 to explain. Furthermore, there are some cases, as in Torenia, where 

 leaf cuttings flower at once almost regardless of the age of the part of the 



' For example, when a Iwig from an apple sapling is grafted on an old stock, it may 

 fruit in a year or two instead of in ten or fifteen years, while a twig from an old stock 

 grafted on a sapling does not fruit for many years. 



