In this article we will discuss about the anatomy of Cycas with the help of diagrams.
Pinnae of Cycas leaves have a single mid-vein without any lateral veins but extending up to the lamina and quite prominent are the sheets of transfusion tissue (Fig. 15.3A, B), which acts as lateral conducting tissue in the leaflet without vein.
A single vascular bundle in the centre is diploxylic — with two distinct kinds of xylem, centripetal as well as centrifugal. The former is roughly a triangular area of xylem with a single protoxylem region. The other is separated from it by parenchyma and consists of an arc of tracheids associated with phloem in such a way as to give the appearance of being secondary in origin.
It is a matter of debate, whether it is to be regarded as secondary wood or a centrifugal primary xylem. However, an important matter is the presence of centripetal xylem, a primitive character reminiscent of protostelic condition. A fibrous sheath surrounds the bundle.
Leaflet of Cycas is characterized by conspicuous xerophytic features. An epidermis of thick-walled, highly cuticularized cells with sunken stomata, confined to the undersurface; 1-2 layered hypodermis of highly cutinized and lignified cells. Present below the hypodermis is a single layer of mesophyll, differentiated into the palisade and spongy parenchyma.
Rachis of the leaf is cylindrical with pinnae inserted on the upper surface (Fig. 15.3B.D forming a shield. The epidermis is of cuticularized cells, interrupted by the stomata (Fig. 15.3C). Hypodermis is a mixture of chlorenchyma and sclerenchyma, followed by the ground tissue in which vascular bundles are arranged in an arc, in the form of an inverted Greek letter omega. (Fig. 15.3D).
Mucilage canals abound the ground tissue, on the upper and lower surfaces of the arc. Mucilage canals, as a characteristic, are common throughout the body of the plant and are of schizolysigenous origin. The mucilage in the canals stores water and checks transpiration, a xerophytic character.
Primary root of Cycas is short-lived and is replaced by adventitious roots which originate as thick (3-9 mm diameter) axes from the base of the stem. At the point of attachment there are 4 to 8 protoxylem points. These roots progressively become thinner from the stem till a diarch condition is reached; the xylem maturation is exarch (Fig. 15.4A).
The vasculature is surrounded by starch-filled pericycle, followed by endodermal cells of casparian strips, a wide parenchymatous cortex and an epidermis. The cambium arises away from primary xylem, so that primary and secondary xylem are separated from each other. Older roots have additional cambia and periderm.
The near-surface roots, particularly those growing in nonxeric condition, turn upward and branch out at the soil surface to form coralloid masses (Fig. 15.4E). These coralloid roots are infected with cyanobiont Anabaena, which occupy an algal zone (az) within the cortex. Apices of coralloid roots have 1-4 cell thick root cap.
Apical meristem is quite massive, it may be as much as 3000-micron-thick in C. revoluta with an organization of primitive kind. The apical initials are a shallow cap of cells which give rise to the central core and flanking region.
Within the core are three regions:
(i) Subapical initial cells, small and densely staining,
(ii) Central-mother-cells, long and lightly staining, and
(iii) Pith rib meristem which gives rise to pith.
The flanking region is derived from the apical initial and partly from central-mother-cells. There is complete absence of tunica in Cycas its cells in the flanking region divide periclinally as well as anticlinally, these are primitive features.
The stem is irregular in outline (Fig. 15.5A) due to persistent leaf bases which provide mechanical strength to the trunk. The wood is conspicuous for its soft texture, surprisingly has small amount of softwood (manoxylic). In the centre is the parenchymatous pith surrounded by a ring of many small vascular bundles, which are followed by wide parenchymatous cortex.
In a transverse section of the stem are seen leaf traces as ‘girdle bundles’. Each leaf receives a number of bundles. The cortical cells are full of starch and are commercially valuable for starchy food hence the plants are often called sago palms.
Secondary growth occurs very early in the life of the plant. The interfascicular cambium develops in the parenchymatous cells between the vascular bundles. The secondary xylem has tracheids with bordered pits and scalariform thickenings, a feature regarded as primitive.
An interesting feature of the stem is that, to begin with, it is monoxylic but becomes polyxylic as a result of accessory rings of cambia, which arise in the cortex. The first vascular cambium as well as the successive cambia become active simultaneously. The number and width of vascular cylinders increase (Fig. 15.5B) from the apex towards the base.