In this article we will discuss about the external morphology of sargassum.
1. Thallus is large, macroscopic and brown-coloured.
2. Plant body is sporophytic and looks like an angiospermic herb.
3. Thallus is differentiated into a basal holdfast, “stem”-like main axis, and primary and secondary laterals. Some of these laterals change into “leaf”- like structures (Figs. 38, 39).
4. Holdfast is irregular, thick and solid structure, and helps in the attachment of plant to some substratum.
5. Main axis or “stem” is cylindrical, erect and flat or smooth structure.
6. From the main axis develops many primary laterals. The primary laterals are spiral in arrangement and show 2/5 type of phyllotaxy.
7. The growth of primary laterals is unlimited.
8. Primary laterals give rise to many secondary laterals, the growth of which is limited.
9. Generally, the secondary laterals become flat and ‘leaf likein shape.
10. A ‘leaf (Fig. 39) is simple, broad and flat structure having serrate, dentate or sometimes entire margin with acute apex. A “leaf’ generally contains a midrib but species like S. enerve are devoid of mid-rib.
11. If a “leaf’ is seen towards the sunlight many dots are clearly visible on it. These are known as cryptostomata or cryptoblasts.
12. Many globular or spherical, air-filled and hollow structures are also commonly seen on the sporophyte. These are known as air bladders (Figs. 38, 39).
13. Air bladders may be present on axis, on primary laterals, at the base of “leaf” or, at the tip of the “leaf”.
14. Air bladders are supposed to help in floatation.
15. From the axil of “leaf’ develops a long, branched system known as receptacle. It bears many flask- shaped fertile conceptacles, containing sex organs.