Question

hydrophytes show reduced xylem.why
?

Answer 1

The development of the transport systems which could accommodate efficient water and carbohydrate transport, was one of the most important requirements for the evolution of an efficient land plant. Long-distance transport becomes a serious problem if inefficiencies exist in either system. Growing larger and taller would also not have been possible without the synthesis of lignin. Emergent land plants evolved over a very long time, with the first vascular land plants evolving during the Silurian about 440 million years ago. The first conifers evolved during the Permian (290-250 my); flowering plants during the late Cretaceous (70-65 my) and the first angiosperms during the late Tertiary.

 

The first vascular plants had fairly primitive vascular systems - these were called protosteles fossil members of the Lycopodiales were at the height of their development and dominance during the Devonian and Carboniferous (408- 360 million years ago). The two examples that we have chosen to illustrate, have  primitive vascular systems (Lycopodium claviatum and L. saururus  illustrate examples of a fairly 'advanced' protostele, one in which the xylem and phloem has become dissected. We illustrate a fern (Rumohra) and then gymnosperms (Encephalartos, Araucaria,  a member of the pines). Common amongst these is their vessel-less xylem. We have included a few examples of angiosperms which have eustelic vascular systems in their primary stems,  (Cucurbit which has bicollateral vascular bundles, Nymphaea a hydrophyte with highly reduced xylem, and a Ligustrum species). Finally, we include Zea mays in which the atactostelic condition and dominant influence of the leaf vascular supply is most evident.

Answer 2

Hydrophytes generally don’t have a need for a well developed vascular system of xylem or phloem tissues. This is because that the plants are constantly surrounded by water, they can absorb the water through their leaves and stems by osmosis, and water can travel throughout the plant by osmosis, therefore they do not require xylem and phloem for the transport of water.