Ramat Bet Hakerem: Semi Shrubs

Published: July 25th, 2010 | Updated: 14/01/15

1.11. Semi-shrubs

Semi-shrubs are perennial plants by definition. Thorough observation all year round reveals that they all have small summer leaves developing at the axile of relatively large winter leaves (Fig. 2.1.32). In this way the plant may be active all year round. It has small leaves in the dry season when the available water is not sufficient for full activity. It may develop large leaves in the season of sufficient water and limited solar radiation. A representative plant is Phagnalon rupestre which has high didactic value. The lower part of a new branch (Fig. 2.1.31 – left) has large winter leaves. The upper part of the same branch (Fig. 2.1.31 – right) displays the part developed toward the summer and hence has smaller leaves with denser hair cover. The entire plant is covered with long white hairs which aid in rejecting solar radiation. The stems in Fig. 2.1.32 have dry winter leaves and small active greenish summer leaves. It is easy, at this stage, to remove the entire wool cover of the stem and see the green outer layer of the stem. The wool may be used to light a fire without matches (Fig. 2.1.32). When the suns rays are concentrated with a magnifying glass and directed onto dry Phagnalon wool, the light is rejected. When a dark dot is marked with a pen or pencil, and the sunlight is concentrated again on the dark dot on the wool, that point starts to smoke immediately.

Fig. 2.1.31: A stem of Phagnalon rupestre which bears large winter leaves, white on their under (abaxial) side and green on the upper (adaxial) side; later (photo on right) the stem developed smaller leaves with denser white cover.

Fig. 2.1.32: A stem of Phagnalon in summer. The large winter leaves are dry, bearing in their axiles small whitish summer leaves.

Fig. 2.1.33: Stems are green below the white hair cover. The “wool” can cause the ignition of fire with a magnifying glass.

Fig. 2.1.34: Inflorescences of Phagnalon.

In this way one may demonstrate the importance of white color in rejecting solar radiation. This phenomenon is repeated in many species of the rock reserve and in the country as a whole. We found a significant statistical increase of white or whitish species along the gradient of annual rainfall from 1000 mm in the Galilee to 100 mm in the Negev. Following the flowering (Fig. 2.1.34), seed dispersal starts, as dealt with in detail in [usefulplantsc3 “Plant Stories” chapter 3, section 3]. When the horizontal dry involucral bracts are wetted, they turn up vertically. Movement velocity increases towards the end of summer. Sarcopoterium spinosum is a common landscape-covering plant in Israel. Three leaf types may be recognized: 1. Winter leaves developing from lower parts of the plant (Fig. 2.1.36, right), 2. Winter leaves accompanying flowers, 3. Small summer leaves having a thick cuticle (shiny). When germinating, S. spinosum produces juvenile bluish leaves covered with wax. The light color of the juvenile leaves assists in the rejection of solar radiation (as demonstrated above). S. spinosum, germinating among annual plants will not succeed in surviving their first summer. The annuals desiccate the root-zone of most of the available moisture. The seedlings in Fig. 2.1.38 were photographed in an area where there was a high-temperature fire, or were under living S. spinosum shrubs. In both sites the competing annuals are weak. When developing with no disturbance, the leafy branches of S. spinosum cover the area and prevail over the annuals by shading that area. As the shrub density increases it covers the entire area (Fig. 2.1.45).

Fig. 2.1.35: After the achene ripen, the involucral bracts are horizontal (left) when dry. After wetting by rain or dew they become vertical and thus closed (right).

Fig. 2.1.36: Sarcopoterium spinosum, on the right – winter leaves on a stem developing from the plant base, developing to form a skeleton branch. On the left – a stem bearing winter leaves and flowers.

Fig. 2.1.37: Stems bearing tiny summer leaves; the xylem hardens and their tips become thorns.

Fig. 2.1.38: Sarcopoterium seedlings that developed in the shade of an adult shrub. The elliptical yellowish-brownish leaves are the cotyledons. The first leaves have a bluish tint, caused by a wax layer covering them, that assists in repelling solar radiation.

Phlomis viscosa is a taller semi-shrub growing in the reserve whose life cycle resembles that of S. spinosum and Ph. rupestre. It has small summer leaves (Figs. 2.1.39, 2.1.40) covered by a thick layer of branched stellate hairs. A cross section of the leaf (Fig. 2.1.40 right) displays a thin green layer with a thick white layer projecting on both sides of the leaf. Other semi-shrubs found in the reserve – Teucrium capitatum, Teucrium divaricatum, and Micromeria nervosa, have a similar life cycle.

Fig. 2.1.39: A Phlomis viscosa shrub growing in a rock fissure, with blackish remnants of fruiting calyces.

Fig. 2.1.40: Summer leaves of Phlomis viscose are relatively small (compare to Fig. 2.1.41-2.1.43). A cross section, at right, displays a thin green layer of the leaf. Two white layers composed of dense branched hairs come out of the green layer.

 

Fig. 2.1.41: Phlomis viscose at the beginning of winter, already having winter leaves.

Fig. 2.1.42: Blooming branches and a dry stem bearing calyces of the previous year (center).

Fig. 2.1.43: Flowers and winter leaves.

Fig. 2.1.44: Sarcopoterium spinosum (at right) prevails over the annuals growing in the other half of the soil pocket, by shading them.

Fig. 2.1.45: A batha of Sarcopoterium spinosum took over the soil pocket shown here. Annuals still grow here and there.