Origin of the summer shoot of pinyon pines

摘要

Shoot development in pines is usually a two-stage process, with buds formed in the summer and elongation delayed until the next spring. Summer shoots have been described which originate by precocious elongation of a bud, a process which may or may not be followed by organization of a new dormant bud. However, in several of the pinyon pines, and perhaps in other soft pines as well, summer shoots are formed without having first been present as a bud. These summer shoots are composed partially of internodes associated with terminal bud scales of the previous year, but mainly of internodes initiated and elongated concurrently in the spring and summer. The major lateral branches of the complex shoot have their origin in the summer shoots, laterals of the spring shoot apparently being inhibited. Short shoots and long shoots of the summer shoot have developmental schedules that contrast with those of the spring shoot.