Underground modifications of stem

1. Tuber :

It is the swollen tip of the underground branch or stolon. The tips become swollen due to accumulation of food material. The tubers are round or oval in shape. Each tuber has many “eyes”, which represents nodes. Axillary bubs are in present in axil of scale leaf. Tubers do not give off adventitious roots; e.g. Solanum tuberosum (potato).

2. Corm :

Corm is a condensed from of rhizome vertically down into the soil. Internodes are usually reduced and one or more axillary bubs are present in the axil of the scale leaves e.g. Crocus sativus (saffron), Colocasia, Amorphophallus (Elephant’s foot) etc.

Rhizome :

It is a thick, prostrate and branched stem growing horizontally beneath the soil surface. It has distinct nodes and internodes. The nodes bear small scale leaves. Axillary bub  is present in the axil of scale leaf.  The lower surface of the nodes gives out small slender adventitious roots e.g. ginger, banana, turmeric.

Bulb :

It is a highly condensed stem on its upper surface are present large number of fleshy leaves surrounding a terminal bub at the centre of the disc. The fleshy leaves of the bulb store food in the form of carbohydrates. A few outer leaves which become dry and scale, are protective in function. The common examples are Allium cepa (onion), Allium sativum (garlic), etc.

Bulb may be Tunicated or Scaly

• Tunicated (Layered or Laminate) : Bulb is covered  with a dry membraneous sheath of scales called tunic. These bulbs may be again of two types :

• Simple tunicated bulb : Onion, Tulipa and Narcissus.
• Compound tunicated bulb : e.g. Garlic

Scaly or  Imbricated or Naked bulb : Tunic is absent e.g., Lily

Stipules

Leaves of some plants have lateral appendages on each side of leaf base, known as stipules.

Petiole :

Leaf is called sessile when there is no petiole e.g. wheat, rice, Calotropis. Petiole bearing leaves are known as petiolate e.g. Peepal, Mango, Guava.

Petioles modifications are

1. Phyllode :

Lamina in some compound leaves falls off soon and petiole gets modified into leaf-like structure and synthesize food e.g. Parkinsonia, Acacia melanoxylon (Australian Acacia).

2. Tendrillar Petiole :

Petioles in some week stemmed plants is modified into tendril and help in climbing of the plants e.g. Clamatis, Nasturtium, Nepenthes etc.

3. Winged Petiole :

It is usually present in compound leaves. In this type petiole becomes flattened and leaf-like and carry out photosynathyesis like the leaf blade e.g. Citrus, Orange.

4. Floating or bulbous petiole ;

In some plants viz. Eichhornia, Trapa etc petiole becomes spongy.