HYPODERMIS
It is also called collenchyma. In Dicot stem it lies below epidermis and consists of 4 or 5 layers of collenchymatous cells. These cells are specially thickened at corners against the intercellular spaces owing to the deposit of cellulose and pectin. The cells are living and contain number of chloroplasts.
In cucurbita stem Hypodermis (collenchyma) lies below epidermis and consist of six or seven layers of collenchymatous cells in the ridge. In the furrows the number of layers is reduced to two or three, sometimes none. In the furrows the underlying parenchyma may be seen to pass right upto the epidermis. Collenchyma contains some chloroplasts.
In Monocot stem Hypodermis is sclerenchyma. This forms a narrow zone of sclerenchyma, usually two or three layers thick lying below epidermis.
(2) TUNICA CARPUS THEORY
Schmit in 1924 proposed this theory for the organisation of the shoot apex. According to this theory, the shoot apex can be differentiated into two regions of unlike structure and appearance, a central core, the corpus with large cells dividing irregularly to result in volume growth. It is surrounded by an outer enveloping layer the tunica. Tunica cells are smaller and divide mainly anticlinically to result in surface growth. The tunica may be one to many layered.
The work of Foster (1939, 1941) and Gifford (1954) gave concept particularly with regard to tunica some workers named tunica to only those layers which never show periclinical division in median position. The additional parallel layers according to them are part of corpus. Other workers treat tunica as fluctuating in number of layers.
According to this view one or more inner layers of tunica may divide periclinically and may become part of corpus. According to Guttenburg (1960) tunica could not consist more than two layers dermatogen and subdermatogen. Sometimes the latter one is not well differentiated.
(3) PARENCHYMA
It consists of a collection of cells which are more or less isodiametric that is equally expanded on all sides. Typical parenchymatous cells are oval, spherical and polygonal in shape. Their walls are thin and made of cellulose and are usually living. Parenchyma is of universal occurrence in all soft parts of plants. Its functions are mainly storage of food material. Parenchyma containing chloroplasts often called chlorenchyma manufactures sugar and starch. Star like parenchyma with radiating arms leaving a lot of air cavities is called aerenchyma as in the petiole of banana and canna also in many aquatic plants.
(4) COLLENCHYMA
This tissue consists of elongated cells with corners or intercellular spaces much thickened with a deposit of cellulose and Pectin. In T.S of stem the cells look circular or oval. Their walls are provided with simple pits here and there. Collenchyma occurs in few layers under the skin (epidermis) of herbaceous dicotyledons e.g. sunflowers and gourd. It is absent from the root and Monocots except in special cases. The cells are living and often contain some chloroplasts. Being flexible in nature collenchyma gives tensile strength to the stem.
Containing chloroplasts it also manufactures sugar and starch. The functions are therefore both mechanical and vital.
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