Diversity of seed plants and their systematics

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pits in several alternating rows on the radial walls. (Fig. 7F) The phloem comprises sieve cells and phloem parenchyma. There are well developed broad medullary rays between the vascular bundles which connect pith with the cortex. The pith is large and consists of Paranchymetous cells rich in starch. It is traversed by numerom mucilage. Secondary Growth:- Secondary growth takes place very early in the life of the plant. The interfascicular cambium develops in the paranchymatous cells between the vascular bundles and joins the intrafascicular cambium. The cambium then cuts off radially seriated secondary xylem towards the pith and secondary phloem towards outside. The secondary xylem shows tracheids with bordered pits and scalariform thichenings. The pittings is multiceriate there being as many as four or five alternate rows of bordered pits which may be oval or round. (Fig. 7F,G) Ultrastructural studies have shown that the torus is absent in the pit memberes of Cycas (Eicke and Metznor- Kuster 1961). The vascular rays are conspicuous and of three kinds viz; (a) uniseriate rays which are 1-10 cells high, (b), multiseriate rays which are 2-5 cells in width and much higher, and (c) foliar multiseriate rays which are continuous from the pith to the phloem. These have a foliar bundle or leaf trace above which is a mucilage duct, usually situated on the phloem side. The ray cells and cortex are full of starch. Calcium oxalate crystals also occur frequently. The phloem is very well developed. In a longitudinal section the individual sieve elements show numerous sieve areas on their radial walls. There are no companion cells, instead certain phloem parenchyma cells, termed as albuminous cells are found closely associated with the sieve cells. The primary cambium is short lived and as soon as the primary cambium has stopped functioning, a second cambium differentiates in the cortex and develops a stronger second ring of wood. Such rings are formed successively one after the other, but the outer rings become, gradually weaker. An interesting feature of the stem is that it is monoxylic to beginwith but becomes polyxylic later as a result of accessory rings of cambia that arise in the cortex. (Fig. 7D,E) According to Terrazas(1991) oservations of the vascular tissue of Cycas shoots, the first vascular cambium as well as the successive cambia become active simultaneously. The successive cambia initiate in the cortical cells. The second vascular cylinder is wider than the first and the subsequent cylinders are narrower than these two vascular cylinders, gradually diminishing towards the peripheny. The number and width at vascular cylinders increase from the apex towards the base (Fig. 7I) Leaf Traces:- A peculiar feature in Cycas is the girdling of the leaf traces (Fig. 7A; 8A,B) there are two types of leaf traces, both arising from the primary vascular bundles. These are girdle traces, and the radial or direct traces i.e. each leaf is supplied by two types of traces. There are two girdle traces which may arise either as two independent bundles or as one single trace bifurcating into tow. These traces arise almost diametrically opposite to the leaf in which they are destined to enter, make a girdle around the stele like two curved prongs of a pair of tongs and become almost completely reversed while negotiating through cortex (Fig.8B) (Making an angle at 140 0 to 150 0 ). The radial or direct traces, on the other hand, take a straight course the cortex. they may bifurcate repeatedly to form a complex network of anastomosing branches. the two girdles traces, joined at intervals by other traces (radial and its branches) so that a number of them enter each rachis. all these bundles exhibit endarch protoxylem and the metaxylem elements are centrifugally placed. (Fig. 9C) RACHIS The rachis is cylindrical, and the pinnae are inserted on it (Fig. 9A). It has a thick-walled epidermis covered by a thick cuticle and stomata are irregularly distributed. There is a broad outer zone of fibrous cells intermixed with short chlorenchyma, and an inner zone of large parenchymatous ground tissue with mucilage canals. Several collateral open vascular bundles are embedded in it. Usually, only two large and a variable number of small strands enter the base of the leaf. Immediately on entering, the two main bundles branch and form numberous bundles which become arranged like the inverted Greek letter omega _ ۟Ώ (Fig. 9A). Towards the tip of the rachis the number of bundles becomes reduced and they become arranged in a C-Shaped arc the centripetal xylem appears like a wedge, with the protoxylem at the apex facing the phloem. An arc of cambium lies on its outer side (towards the phloem). A few centrifugal xylem elements are present on its inner side, usually in two or three separate groups or in a continuous arc. The centrifugal xylem is separated from the protoxylem and adjacent centripetal xylem by a few parenchyma cells. On its outer side, towards the primary phloem, a few 3
layers of radially seriated secondary phloem cells are present. Each vascular bundle is surrounded by a sclerenchymatous sheath. This characteristic diploxylic bundles are seen in the rachis as well as in the pinnae. The xylem elements are endarch or centrifugal at the base of the petiole. After travelling a certain distance in the rachis, centripetal metoxylem elements appear on the inner side of the protoxylem. Concurrent to appearance of centripetal xylem, formation of centrifugal xylem is reduced considerably so much so that the vascular bundles towards the tip of the rachis shows only a few centrifugal xylem tracheids. thus at higher livel the centripetal xylem is more abundant than the certrifugal xylem (Fig. 9C) such vascular bundles are termed as pseudomesarch or diploxylic (Fig. 9D, E) as they show the presence of both centripetal and centrifugal xylem. As the bundles move up into he pinnae, they become exarch with no traces of centrifugal xylem. this condition is however rare. LEAFLET:- The leaflet is dorsiventral has xerophytic characters (Fig. 10 A-C). The upper and lower epidermis are covered by thick cuticle. The epidermis is single layered end its cells are lignified the upper epidermis is continuos but the lower epidermis is interrupted by the presence of sunken stomata which are haplocheilic. Internal to the upper and lower epidermis lies, a single layered thick sclerenchymatous hypodermis which in the region of midrib is two or more layered thick. Next to hypodermis is a zone of chlorophyllous mesophyll cells. The mesophyll is differentiated into a vertically elongated palisade tissue situated towards the upper side and loosely arranged spony parenchyma tissue towards to lower epidermis. In between the palisade and spongy paranchyma occur three or four layers of lignified tracheid like colourless empty cells, showing borders pits on their walls, which run parallel to the leaf surface from the midrib to the margin. It constites the accessory transfusion tissue. It probably functions as lateral conducting tissue in the leaflet which lacks veins. In the midrib region there is a single large vascular bundles surrounded by jacket of thick walled cells. It consists of xylem facing the upper surface and phloem facing the lower surface in the form of an arc (Fig. 10B-C). The xylem strand (centripetal xylem) appears like a wedge with the protoxylem placed at its apex facing the phloem and arc of cambium lies on its outer side i.e. towards phloem. on either side of protoxylem are present two or three tracheids centrifugal xylem in separate groups. the latter is separated from former by a few parenchyma cells. The vascular bundle is diploxylic showing pseudomesoarch condition. (Fig. 10 A, B) Transfusion tissue consisting of transfusion tracheids, transfusion parenchyma and albuminous cells, occurs on the sides of the vascular bundle. On the sides of the yxlem transfusion tracheids with bordered pits are present which are connected on either side by accessory transfusion tracheids. Xerophytic Features:- 1. Highly cuticularized epidermal cells which are thick walled. 2. Lignified hypodermal cells. 3. Sunken stomata 4. Unbranched midrib and occurrence of transfusion and accessory transfusion tissue THE REPRODUCTIVE CYCLE Cycas revoluta is extensively propagated vegetatively by means of adventitious buds or bulbils. The male plants of C revoluta are not found in Northern India. Hence no seeds of this species are available in this part country. The female plants are raised entirely by means of bulbils. The male plants of C. circinalis are relatively more common. All Cycas spp are dioecions. In the vegetative state the male plants are indistinguishable from the female plants. The appearance of the male and female reproductive structures distinguishes the two sexes. The reproductive cycle starts after several years of vegetative growth. MALE CONE The male cones are borne singly and terminally on the main stem (Fig. 11A). The apical growth of the stem is continued by the formation of the axillary bud at the base of the cone. It becomes the new stem apex. As it grows the male cone is displaced to one side. The new shoot apex soon takes over forming a crown of leaves 4
Page 1 and 2: Diversity of seed plants and their
Page 3: The Primary root system is a tap ro
Page 7 and 8: small as compared to tube cell and
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Diversity of seed plants and their systematics

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