To perform some secondary functions, roots undergo modifications. They are identified as tap root due to their origin from radicle and bearing root hairs.
1. Fleshy/storage Tap Roots. Tap roots become swollen due to storage of food. Hypocotyl may also become enlarged and swollen. Secondary roots are thin and thread-like. They do not occur in the region of hypocotyl. These roots arise singly. The leaves are radical as they appear to arise from root directly.
(i) Conical: Fleshy root is like a cone, being thickest towards the base (soil surface) and gradually tapering towards apex. Thread like secondary roots are found throughout the length of the root showing that fleshy part is tap root e.g., Carrot.
(ii) Fusiform: The root is swollen like a spindle, being thickest in the middle and narrowing towards both apex and base. In Indian Radish only the base is formed of swollen hypocotyl. In English Radish the basal half is swollen hypocotyl while the apical half is swollen enlarged tap root Lei, edible part is formed by hypcotyl and tap root both. Here less than 2/3rd part of root is formed by hypocotyl. Secondary roots develop from the tap root part only.
(iii) Napiform is like a ‘top’ or sphere that thins out abruptly at the apex; ln Turnip, the swollen part is hypocotyl. ln beet chakundar/red root the swollen part is formed jointly by hypocotyl and tap root. Secondary roots occur in tap root part. More than 2/3rd part of fleshy root is formed by hypocotyl.
(iv) Tuberous: Tap root is thickened except at the base without producing a definite shape, e.g., Mirabilis.
2. Nodulated (Tuberculated roots). ln legumes (e g., Pisum sativum, Cicer arietinum) the roots bear numerous irregular swellings called nodules or tubercles. The nodules are pinkish and contain nitrogen fIxing bacteria Rhizobium Both bacteria and legume are benehtted by this association called as Ciprocal symbiosis. Due to it legumes are rich in proteins and are used in crop rotation. Nodules are Dinkish due to a pigment leghaemoglobin which carry oxygen and provide anaerobic environment in nodules tor nitrogenase to fix nitrogen. These nodules are found on primary and secondary roots only.
3. Pneumatophores: Pneumatophores or respiratory roots are short, vertical and negatively geotropical (apogeotropic) roots which occur in mangrove plants (Halophytes of swampy saline areas near sea shores). The roots come out of swamp; Their upper ends bear lenticels (pneumathodes) for Exchange of gases, e. g. Avicennia, Sonneratia,
4. Buttress Roots. They are laterally compressed horizontal roots which travel along the ground , f0f some distance and provide extra mechanical support 9 g., Ficus elastica (Rubber Tree) Ficus religiosa Peepal) Bombax (Simbal) These roots are formed jointly by base of trunk and tap root.
5. Reproductive Roots. Some tap roots or their branches develop adventitious buds and help in Vegitative reproduction, e. g., Daibergia, Populus
6. Mycorrhizal roots. These roots of higher plants possess fungal hyphae (live symbiotically) which help them to absorb food as well protection against other fungi .Root hairs are absent. Fungal hyphae may act as root hairs. e.g. Pinus, Bitula
No comments:
Post a Comment