Читать книгу Lichens - Annie Lorrain Smith - Страница 18

Table of Contents

Fig. 1. Physcia aipolia Nyl. Vertical section of thallus. a, cortex; b, algal layer; c, medulla; d, lower cortex. × 100 (partly diagrammatic).

The thallus or vegetative body of lichens differs from that of other green plants in the sharp distinction both of form and colour between the assimilative cells and the colourless tissues, and in the relative positions these occupy within the thallus: in the greater number of lichen species the green chlorophyll cells are confined to a narrow zone or band some way beneath and parallel with the surface (Fig. 1); in a minority of genera they are distributed through the entire thallus (Fig. 2); but in all cases the tissues remain distinct. The green zone can be easily demonstrated in any of the larger lichens by scaling off the outer surface cells, or by making a vertical section through the thallus. The colourless cells penetrate to some extent among the green cells; they also form the whole of the cortical and medullary tissues.

Fig. 2. Collema nigrescens Ach. Vertical section of thallus. a, chains of the alga Nostoc; b, fungal filaments. × 600.

These two different elements we now know to consist of two distinct organisms, a fungus and an alga. The green algal cells were at one time considered to be reproductive bodies, and were called “gonidia,” a term still in use though its significance has changed.