122 • Age and Flowering 



been studied relatively little because of the obvious technical 

 difficulties. Increased soil fertility may be of value (for experiments 

 that deal with this possibility using pine, see Hoekstra and Mergen, 

 1957). Fraser (1958) has correlated meteorological data with anatom- 

 ical studies of spruce (Picea), and concluded that earlier reports 

 that flower initiation is favored by high summer temperatures are 

 probably correct. Reference to the discussions in the papers cited 

 will indicate that, unfortunately, tree physiologists are generally 

 uncertain about the importance of any particular soil or climatic 

 factor. 



Photoperiodism affects largely the vegetative development of 

 woody plants rather than flowering, at least according to present 

 evidence. The rate of growth, its cessation and renewal, branching 

 habit, leaf shape, and resistance to cold are among the characteris- 

 tics affected (see Wareing, 1956; Nitsch, 1957). Such characteristics 

 are often of great ecological significance, and their sensitivity to 

 photoperiod frequently differs considerably within offspring of the 

 same species gathered over a wide geographical area (see Vaartaja, 

 1959). In certain crop trees, such as the SDP Cofjea arabico (coffee), 

 flowering also is photoperiodically controlled (Piringer and Borth- 

 wick, 1955), whereas the ornamental shrub Cestrum nocturnum has 

 been previously discussed as an LSDP. 



Most work with economically important trees, however, sug- 

 gests a minor role or none at all for photoperiodism in flower 

 initiation. This is almost certainly true for pines (Mirov, 1956; see 

 Mirov and Stanley, 1959), for peaches (Prunus), and probably for 

 apples (Mains) (Piringer and Downs, 1959). One should note an 

 indication of control by light in the last-named tree, however. In 

 the paper cited, the variety used failed to flower at all on 16-hour 

 photoperiods of which 8 hours were under fluorescent light, but 

 flowered well if incandescent light was used. For such reasons, as 

 well as because of the relatively few experiments done so far, it is 

 impossible to guess whether or not photoperiodically controlled 

 flowering is truly less common among woody plants than it appears 

 to be among herbs. Certainly, however, even when photoperiodism 

 is a direct factor, that of size or maturity is still of overriding 

 interest both practically and theoretically. 



Because of effects on vegetative growth, photoperiodic treat- 

 ment can indirectly hasten flowering. A species of birch, lietula 



