NEET SHORT NOTES & TRICKS:
❇️ STEAM ❇️👉Stem develops from the plumule of the embryo of a germinating seed.
👉The stem bears nodes and internodes.
👉The region of the stem where leaves are borne are called nodes while the portions between two nodes are
called internodes.
👉Stem shows negatively geotropic growth.
👉The main function of the stem is spreading out branches bearing leaves, flowers and fruits.
👉 It conducts water, minerals and photosynthates (food).
👉Some stems perform the function of storage of food(potato, ginger, turmeric, zaminkand and Colocasia), support, protection and of vegetative propagation.
(1)Runner - In these stems roots develop at lower side and leaves at upper side from the nodes eg. Cynodon dactylon (doob grass)
(2)Stolon - In it branches develop from the lower part of the main stem and grow for some distance like arch and finally touch the ground to give rise to new shoot. Eg jasmine
(3) Sucker - In it the main stem grows in the soil but branches develop from basal and underground nodes and comesout from the soil. Eg. Mint, pineapple, Chrysanthemum, banana.
(4) Offset - Generally these occur in aquatic plants which have fragile stem. Internodes of offset are small & thicker and
each node bears a rosette of leaves and a tuft of roots. It is also known as aquatic runner. Eg. Pistia, Eichhornia
(5).Bulb - Stem is highly reduced and disc like and surrounded by numerous fleshy leaves. Many roots arise from its base. Eg. Onion, garlic. The fleshy leaves of onion and garlic store food
♻️ Organ of perennation - Underground stems of potato, ginger, turmeric, Colocasia, zaminkand are modified to store food in them. They also act as organ of perennation to tide over conditions unfavourable for growth.
(1) Stem tendril - In this type of modification axillary bud forms tendril in place of branches and helps in climbing of those plants which have weak stem. Eg. Grapes/Grapevines, Passiflora, gourds (cucumber, pumpkins, watermelon)
(2) Stem thorn - Thorn develops usually from axillary bud of the stem. It may bear leaves, flowers.
Eg. Carissa (karonda), Bougainvillea, pomegranate,Citrus
👉Thorn protects plant from browsing animals. It is a woody pointed structure.
(3) Phylloclade - Stem is modified into a fleshy flat (Opuntia) or fleshy cylindrical (Euphorbia) and green leaf like structure and carries out photosynthesis like leaf
