In this article we will discuss about the Coniferous Leaves. After reading this article you will learn about: 1. External Morphology of Coniferous Leaves 2. Anatomy of Coniferous Leaves.
External Morphology of Coniferous Leaves:
Laubenfels (1953) recognised four distinct types of leaves among the living conifers:
1. Needle-like leaves having a single midrib and appearing as a three-sided or four-sided structure in transverse section, as in several living genera of Araucariaceae, Pinaceae (e.g. Abies, Cedrus, Pinus), Podocarpaceae, etc.
2. Bifacial, lanceolate or linear leaves having a single midrib, e.g. Araucaria.
3. Photosynthetic or non-photosynthetic scale-like leaves as in Cupressaceae (e.g. Cupressus sempervirens).
4. Broad and multi-veined leaves as in Podocarpus and Agathis.
Coniferous leaves, in general, are either scale-like or have well-developed green laminae. In most of the genera they are simple, evergreen and uni-nerved. They may sometimes be as long as 30cm (e.g. Podocarpus).
Usually the leaves are linear and needle-like (e.g. Pinus) but in some genera (e.g. Agathis and Podocarpus) they are broad and possess open dichotomous venation. In several members (e.g. Araucaria) the leaves persist for 10 to 15 years but in Larix, Metasequoia and Taxodium the leaves are deciduous and fall in almost every autumn.
In majority of coniferous genera, the leaves are arranged alternately or spirally. But in Cupressaceae the leaves are either arranged in opposite (e.g. Callitris, Calocedrus) or whorled (e.g. Fokienia, Fitzroya) manner.
In Pinaceae (e.g. Pinus, Abies) the leaves are borne only on dwarf shoots but in Cupressaceae the leaves develop always on long shoots. In certain Pinaceae (e.g. Pseudolarix), however, the foliage leaves develop both on dwarf as well as long shoots.
The number of needles or foliage leaves on a dwarf shoot differs in different species of Pinus and ranges from one (P. monophylla) to five (P. wallichiana). In genera such as Cupressus and Thuja the scaly leaves are also green in colour.
Anatomy of Coniferous Leaves:
A thickly circularized, well-developed epidermis covers the leaf all over. Continuity of the epidermis is broken by several sunken stomata. They are of haplocheilic type. Stomata are hypostomatic i.e. found only on the abaxial surface of the leaf in several genera including Podocarpus, Cephalotaxus, Tsuga and Abies.
In Juniperus, Thuja and Cupressus, however, they are hyperstomatic i.e. found only on adaxial surface of the leaf. In Pinus and several other members of Pinaceae they are found on all sides of the leaf. There is, however, no definite arrangement of stomata in genera such as Cryptomeria and Cedrus.
A sclerenchymatous hypodermis is present in the leaves of several genera including Agathis (Fig. 11.1), Abies, Cryptomeria and Picea. Hypodermis occurs only in distinct patches in several species of Pinus. In the leaves of Tsuga and Cephalotaxus, however, hypodermis is absent.
Mesophyll is well-developed and remains differentiated in palisade tissue and spongy parenchyma in several members such as Podocarpus and Agathis (Fig. 11 1). In some other genera, however, mesophyll is undifferentiated.
Branching sclereids are present in the mesophyll tissue in Sciadopitys (Taxodiaceae). Cells of mesophyll show infoldings in the needles of Pinus (Fig. 11.25). Well-developed air spaces are present in the mesophyll region of leaf in Cryptomeria japonica (Fig. 11.2).
Each vascular bundle remains surrounded by a well- developed endodermis. In several genera (e.g. Cryptomeria, Cunninghamia, etc.) a big resin canal is present above the vascular bundle (Figs. 11.2, 11.3). In Pinus, the resin canals are present throughout the mesophyll tissue (Figs. 11.24, 11.25). In Picea excelsa (Fig. 11.4) a single resin canal is present in the assimilatory tissue.
One of the characteristic features of Coniferous leaves is the presence of transfusion tissue.
It usually consists of:
(i) Transfusion tracheids made up of dead cells containing thick and lignified walls, and
(ii) Transfusion parenchyma cells, which are living and contain tannin.
Main function of transfusion tissue is to conduct water and food materials, chiefly between mesophyll tissue and vascular bundle. In the lower regions of the leaf of Araucaria, xylem of the vascular bundle remains completely surrounded by the transfusion tracheids.