Histogenetic patterns and classification of the shoot-axis galls
On the basis of observations made on more than 50 shoot axis galls on different host plants caused by fungi (in 2 plants), mites (in 4 plants) and insects (in 48 plants), the responses of different tissues of the stem to the cecidogenetic stimuli were elucidated. The analysis lends itself to the fact that the behaviour of various tissues of the shoot axes seems to be correlated to the initial position of the cecidogenetic agents. When the gall inducing organism is external on the stem, the epidermis or the outer cortex proliferate to form ‘covering growth’ gall; when the cecidogen is in the cortex, the entire cortex forms a ‘rinden’ type of gall. Likewise, the phloem, cambium, xylem and pith proliferate contributing primarily to the cecidogenesis when the cecidogen is positioned in these tissues. On the basis of the position of the gall inducing organism in the gall, and the tissues that contribute solely or primarily to the gall development, a classification of the shoot-axis galls is proposed.