The cell wall and the protoplasm are both freely permeable to water and indeed the former is also permeable to the molecules of crystalloid substances. The living protoplasm, however, may be described as semi-permeable since it may restrict the passage of dissolved substances either into or out of the vacuole. One result of this property of the protoplasm is that, whereas at a given moment the pressure of molecules of water inside and outside the cell may be equal, there may exist in the vacuole in addition a pressure due to the molecules of the salts; the cell is in conse quence distended, the wall is stretched and owing to its elasticity exerts a counter-vailing pressure. Such a cell is described as turgid and in this condition it acquires a certain rigidity analogous to that of a distended air bladder.
This characteristic of the vegetable cell has played an important part in the evolution of the plant body since it has rendered pos sible the development of plant organs of relatively large size with a minimum organization of skeletal tissue. Such structures as the leaves of herbaceous plants—though provided with a network of veins—maintain their form and position only by the turgidity of the living cells of which they are largely built up. Loss of water from the individual cells results in a shrinkage of the tissues as a whole, and the familiar appearance of wilting or flagging of plants deprived of water is brought about in this way.
The plastids or chromatophores are portions of the protoplasm which have become specialized for the performance of certain functions. They are readily distinguished in the adult cell by their size and definite form and they are often conspic uously coloured. Leucoplasts are colourless and occur in the cells of the deeper lying tissues where light cannot penetrate—if ex posed to light they often become green. They are frequently con cerned with the formation of starch grains. Chromoplasts are colour-carrying bodies giving red and yellow colours to the petals of flowers, ripe fruits, etc. ; they contain yellow and red pigments such as xanthophyll and carotin. The most important of all the plastids, however, are the green chloroplasts, chlorophyll bodies or chlorophyll corpuscles as they are variously termed. They contain chlorophyll and are of universal occurrence throughout the vegetable kingdom with the sole exception of the important group of the Fungi. It is by means of this pigment that the plastids are enabled in the presence of light to form carbohydrates from water and carbon dioxide.
The importance of this process cannot be overestimated, since it is the starting point in the manufacture of the food of all living organisms and its activity has made possible the existence of plants and animals upon the earth. (See PHYSIOLOGY.) Chloro phyll when extracted from plants by suitable solvents can be shown to contain a number of pigments. The green, blue-green, yellow and red have been already mentioned. In form the chloro plasts are extremely variable and the Algae (q.v.) show a rich variety of shape ; in the higher plants however they are discoid in form. Like the leucoplasts they frequently show inclusions of starch grains. They may increase in number by dividing into two halves by simple constriction. They have been shown to be present in the egg cell of some plants and are, in these cases, thus passed on from one generation to another, but they have not been demon strated in the sexual cells of all plants.
The cell wall in meristematic cells is an extremely delicate membrane but, with the completion of the growth in size of the cell, it is increased in thickness and may in certain cases undergo considerable modification. It is commonly regarded as a secretion of the protoplasm and it is in living cells in intimate relation with the protoplasm which may interpenetrate the particles of its substance. This close relation can be readily appreciated when the process by which the wall is first laid down is considered. From the beginning of the plant's existence as a single cell, all new cell formation consists in the division of a pre existing cell into two halves by the formation of a dividing wall. In this process the cytoplasmic spindle which functioned in the preceding nuclear division (see CYTOLOGY) plays an important part. The spindle fibres—increased in number—show swellings or thickenings in the middle of their length (fig. 1, B), which coalesce and form a continuous "cell plate" which is extended until it reaches the side walls of the mother cell. This cell plate is not however the cell wall; it is of protoplasmic nature and splits into two layers between which the true wall substance is laid down (fig. 1, D). The subsequent growth of the cell wall is facilitated by its intimate penetration by the protoplasm, which lays down fresh materials between the existing particles (intussusception), and later adds to its thickness by the deposit of fresh layers of material on the inner surface of the primary wall (apposition).