Analysis of shoot apical organization in six species of the Cupressaceae based on chimeric behavior.

Six species of the Cupressaceae, the variegated Leyland cypress (Cupressocyparis leylandii 'Silver Dust'), savin (Juniperus sabina variegata Laws), davurian juniper (Juniperus davurica 'expansa variegata'), California incense cedar (Calocedrus decurrens 'Aureovariegata'), the American arbor vitae (Thuja occidentalis 'lutae zebrina' Kent), and the sawara false cypress (Chamaecyparis pisifera 'nana aureovariegata') were examined for the behavior of albino-green shoot chimeras. The fate of the variegations in these six plants is the same in two important respects. First, the majority (89%) of sprays with an original sector become completely white. Second, sectorial branch sprays of the original sectorial sprays become either completely green or white in a 1 : 1 ratio. Based on the first finding it is concluded that there is one rather than the two to four apical initials in the shoot apex, as generally postulated. This single apical initial, actually an apical cell lineage, residing in the tunica layer can both form the leaf epidermis and by rare periclinal divisions form sectorial chimeras. The second finding is that there is no selection advantage of either type, a feature also postulated by others.