Ferns store an abundance of starch in the cortex of the rhizome and temporarily in the palisade cells of the fronds.
Flowering plants during favourable conditions of sunlight make and store starch temporarily in the photosynthetic layers inand young . During the night this starch is reconverted into sugar and transported to places where it is used or is converted into a storage substance, to be stored in a storage tissue or organ. Some of it is continually utilized in the to make proteins, and protein synthesis can occur in darkness. The form in which food is stored can be carbohydrate, lipide or protein, depending upon the part of the plant where it is stored, and upon each species of plant. In many cases the quantity of material stored for future use is so great that the particular structure assumes a characteristic shape and may prove a useful source of food to man and other animals. For the sake of brevity and clearness these structures have been tabulated, and their nature will be discussed afterwards.
Many fruits andcontain oils of economic importance, e.g. olive oil from the fleshy fruit-wall of the olive, coconut oil from the ripe of the coconut palm, castor oil from ricinus, linseed oil from lin seeds and cotton- oil.
Before attempting to determine what the structures are in the foregoing table, it is necessary to consider the characteristic features of aand a shoot. Externally a root normally bears no structures other than side , with root hairs and a root cap in the youngest part, whereas a shoot bears leaves at nodes with buds in their axils, the terminal bud occupying a unique in that its growth gives rise to an extension of the shoot and not to a branching of it. The presence of buds and leaves will fix a structure as being a and not a root. Internally a root consists of a central cylinder surrounded by the cortex, whereas a stem of a dicotyledonous plant possesses well-marked vascular bundles surrounding a well-defined pith area, and surrounded by the cortex, whilst a monocotyledonous plant has its vascular bundles evenly scattered.
In a cross section, therefore, the presence of two distinct areas denotes a root; of three areas a dicotyledon stem ; and of scattered vascular bundles a monocotyledon stem.
A vertical section of the cabbage or brussels sprout will show the swollen main axis with a large pith area bearing closely packed leaves with buds in their axils. Since the internodes are extremely short, the structure must be a shoot with a telescoped stem. Examination of any bud will show a similar arrangement of parts, so that the cabbage and brussels sprout must be ’enlarged winter buds, forming a part of a plant. ’
If we take dormant bulbs to pieces we find them to be made up of thin sheets of dry, brown fibrous material, surrounding thick fleshy whitish structures, both kinds being attached to a disc, the under side of which bears withered fibres and small bulging structures which will grow into next year ’s roots. Between the fleshy structures will be found tiny buds, and, in a somewhat central, a bud or the remains of last year ’s shoot. The presence of buds fixes the nature of the disc as a stem, and the structures attached to its upper side as withered protective leaves and swollen food-storage leaves respectively. Since they are closely packed on the disc, we may regard this as a telescoped stem. This corresponds to our definition of a bud as given above, and we may therefore define a bulb as being ’an underground bud with fleshy leaves on a telescoped stem bearing adventitious roots beneath, ’ and the whole is therefore a complete plant. In the Tulip bulb the food is stored in swollen scale leaves, but in the majority of bulbs, e.g. the , it is stored in the fleshy swollen bases of the leaves of the previous year.
The external structure of a rhizome such as Solomon ’s seal shows a withered portion at one end, with a very distinct bud at all other ends if the structure is branched. Running across are thin fibrous structures, which are clearly scale leaves or scars left by them, and near the terminal bud is another bud, which shows the whole structure to be a swollen stem growing horizontally beneath the surface of the soil, for this bud lies in the axil of a scale. It will be noticed that adventitious roots arise at the nodes which are denoted by the scale leaves or their scars. Somewhere along the length a circular scar of last year ’s aerial growth is to be found. This shows the arrangement of scattered vascular bundles characteristic of a monocotyledon stem.
A rhizome is therefore a ’complete plant consisting of a swollen horizontal underground stem bearing prominent terminal buds, scale leaves, axillary buds and adventitious roots. ’ The stored food is evenly dispersed throughout the stem.
A corm bears all the parts found on a rhizome, but is roughly spherical and may be one of a string growing vertically. From the base of the uppermost corm thick roots are sometimes to be found. These go deep into the soil. They are contractile, pulling the corm above down below the surface of the soil. Since a new corm is formed each year on top of last year ’s, the advantage of this is obvious. A corm, then, is ’a complete plant consisting of a swollen vertical underground stem bearing axillary buds, adventitious and contractile roots, and enclosed by brown fibrous scale leaves. ’ A vertical section shows the food to be stored in the cortex.
At one end of a potato tuber is to be found a thin, dried, dead fibrous structure where the tuber was formerly joined to the parent plant. At the opposite end to this is a bud. At each ’eye ’is to be found a bud, generally flanked by a bud on each side, just below which is a vestigial scale leaf. In the case of the Jerusalem artichoke the terminal structure is more obviously a shoot, and the scale leaves are more clearly seen, bearing in some cases bulges which are immature buds. A tuber is a complete plant and is ’a swollen end portion of an underground stem. ’ If a section be cut through the region of the eye in a potato the vascular strands can be seen, and their position shows that most of the food is stored in the pith area. The tuber of the Jerusalem artichoke bears adventitious roots at the nodes, but the roots in a potato are produced when it begins to sprout at the bases of the new shoots.
The nature of a tap root is most readily determined bya vertical section of it. Running up the middle will be seen definite strands which separate near the top to enclose a softer area there. This softer region is clearly a pith area, so that, apart from this, a tap root is ’a swollen main root with a telescoped stem at the top bearing a terminal bud surrounded by a mass of leaf bases ’. Since the strands which form the central cylinder do not occupy much of the root, the food is stored in the cortex. In the beetroot there are concentric rings of water conducting xylem and phloem where most of the food is stored.
Plants such as the dahlia and lesser celandine do not store food in the main root but in a number of roots, which thereby become swollen and resemble tubers when massed together, and are therefore termed tuberous roots. Their internal structure shows them clearly to be roots.
Many plants die down with the approach of winter, but the parts below the surface which may not show any pronounced storage organs sprout again the following year. These plants include the herbaceous perennials like the, growing up each year from a root stock.