733 



SECRETIONS OF PLANTS. 



SECRETIONS OF PLANTS. 



731 



animals, where neither bloodvessels nor basement membranes are 

 present, the cell has the power of forming special products, and sepa- 

 rating them from the fluid by which it may be surrounded. [CELLS; 

 GLANDS; MEMDRANE; INTESTINES.] 



On the surface of serous membranes a serous liquid is formed, which 

 is supposed to be simply an exudation or oozing from the vessels 

 supplying the membrane. Whether such exudations are entitled to 

 the name of secretions is doubtful. Various circumstances influence 

 the function of secretion in the higher animals. Among these may be 

 mentioned : 



1. The quantity of blood. 



2. The composition of the blood. 



3. The state of the nervous system. 



An increased supply of blood generally increases secretion. At the 

 same time an increased secretion from a part will draw a larger 

 quantity of blood to a part, so that these conditions react on each 

 other. Tlipre are however diseased conditions in which an increased 

 supply of blood to an organ suspends altogether secretion from a part. 

 This is the case in some stages of inflammation of the mucous 

 membranes. 



That secretion is dependent on the supply of materials in the blood 

 is frequently observed. Thus when one kidney is diseased the blood 

 gets a larger supply of urea, and the other kidney throws off a much 

 larger quantity than it otherwise would do. The kidney also grows 

 larger, and this indicates the relation between nutrition and secretion. 

 At the mirne time in diseased conditions we have exceptions. In gout 

 the kidneys seem unable to carry off the lithic acid from the blood, 

 whilst in rheumatism the lithic acid is so freely carried off that none 

 ia found in the blood. 



The nervous system in the higher animals exerts a powerful influence 

 on the secretions. Excitement of the mind will produce tears. Fear 

 and anxiety increase the intestinal and urinary secretions. Injuries 

 to the spinal cord increase the phosphate in the urine. The thought 

 of food increases the flow of saliva. The mother's breast fills with 

 milk at the thought of or sight of her infant. Under the influence 

 of passion or fear the milk becomes changed in composition, and 

 injurious. 



The purposes which the secretions serve are various. Some, as 

 already stated, have no other purpose than to moisten the several 

 parts of the body; others are appropriated to the service of important 

 functions, as the saliva, the gastric juice, the bile, and the pancreatic 

 and intestinal fluids, which all assist in digestion. Such also are the 

 tears, which perve for the cleansing of the front of the eye ; the milk 

 for the food of the offspring ; and various materials for the defence 

 and comfort of the animal, and for the propagation of the species. 

 Other secretions again, which are commonly called excretions, are the 

 refuse of the blood, and if retained in it would exercise an injurious 

 influence on the whole economy ; such are the cutaneous perspiration, 

 the urine, and those parts of the bile and intestinal fluids which are 

 not used in digestion. [BILE; GLAND; INTESTINES; LIVER; MAMM.VIIT 

 GLAND; MILK; Mucus; PANCREAS; SALIVA; SKIN; STOMACH.] 



SECRETIONS OF PLANTS are those substances which are found 

 in Plants, and which have been formed by the action of the cells upon 

 the compounds which have been taken up into the plant as food. 

 [SAP.] From this point of view all substances, whether composing a 

 part of the tissue of plants or thrown out upon their surface, are 

 regarded as secretions. 



Although the term secretion is generally connected with the idea 

 of separating for the purpose of throwing off, or getting rid of a 

 product, it is very manifest that such a use of the term would 

 restrict its application to the substances which, amongst animals, 

 are called excretions. It does not appear that any one class of sub- 

 stances can be called excretions more than another in the vegetable 

 kingdom. It is true that a theory of the practice of 'rotation of 

 crops ' supposes it to depend on poisonous excretions given off by 

 the roots of one plant which are not poisonous to another. But the 

 facts brought forward to support this theory are doubtful, and other 

 explanations of the necessity of rotation have been given. [HooT.] 



In plants the organs of secretion are simpler than those of animals, as 

 they have no fixed reservoir from which to draw the materials of secre- 

 tion, an the blood. This function however seems to bo performed in 

 both plants and animals on the same general plan. It is in both cases 

 in the interior of the cell that the most remarkable instance of the 

 process takes place. In the plant the compounds changed are simpler, 

 whilit the chemical forces in action during secretion are stronger than 

 in animals. AH the important secretions of plants are compounds of 

 the four organic elements : carbon, hydrogen, oxygen, and nitrogen. 

 These enter the plant in the form of carbonic acid and ammonia. 

 Out of these compounds the various substances that give the hard- 

 ness to the wood of plants, the nutritive value to their seeds, roots, 

 and other parts, the colour and ecenta of their leaves and flowers, with 

 the medicinal virtues of many special plants, are formed. The sub- 

 stances thus produced are easily distinguishable, and may be divided 

 into two great classes. 



First, Nutritive or Assimilable Secretions, that is, substances which 

 having been formed in the plant, are used for forming its tissues, and 

 constructing the mass of which it is composed. The principal sub- 

 stances which are thus employed are cellulose, starch, sugar, oil, and 



protein. The first four are distinguished by containing the elements 

 carbon, hydrogen, and oxygen, whilst the latter contains in addition 

 nitrogen. [CELLULOSE; DEXTRINE; STARCH; SUGAR; OIL; PROTEIN.] 



These substances are found universally in the vegetable kingdom. 

 No cell can be formed without one of the ternary compounds, and a 

 portion of the quaternary substance in some form. Hence they are 

 called in relation to the plant Nutritive Secretions. These substances 

 are also easily convertible the one into the other ; the sugar may be 

 converted into starch or cellulose, and vice versa, and thus their 

 powers and properties are essentially connected with the assimilative 

 processes of the plant. 



The second class of substances are called Non-Assimilable or Special 

 Secretions of Plants. They are substances which are not found iu 

 every part of every plant. When once formed also they are not 

 liable to change, and are certainly never converted into the nutritive 

 secretions ; hence they are called non-assimilable. Some of these sub- 

 stances are very generally diffused amongst plants, as chlorophyle, 

 which is the substance which gives the peculiar green to the leaves 

 and other parts of plants. [CHLOROPHYLE.] 



These secretions are very numerous, and may be classed under 

 certain general heads. 



1. Colouring matters. To this head may be referred chlorophyle ; 

 the colouring principle of the petals of plants seems also to be a 

 modification of this substance. There are however other colouring 

 matters in plants, such as those used by the dyer, and which do not 

 give any colour to the plants iu which they exist, which have never- 

 theless a very definite chemical composition, and by combining with 

 various other substances produce the colours used by the manufac- 

 turers of coloured cotton, linen, silk, and woollen cloths of various 

 kinds. These colouring matters would appear to arise from the 

 decomposition of the assimilable secretions, as many of them bear a 

 close relation to both the ternary and quaternary forms of these 

 secretions. 



2. Acids. Substances having an acid reaction, and capable of 

 combining with the oxides of the metals, are very common in the 

 vegetable kingdom. The most familiar forms are those which occur 

 in fruits, as the oxalic, citric, malic, and tartaric acids. Oxalic acid is 

 found in the Ozalis Acetosella, hence its name, and other forms of 

 O^alidacea. It is also found in the Cactacete and Polyyonacece. In 

 the latter order it exists in the species of Rheum (Rhubarb), used for 

 making pies, and also in the Sorrels (Kumex). In all these cases it is 

 combined with the oxide of some metal, either potassium or calcium. 

 In Sorrel (Rianex acetota) it exists as a quinoxalate or superoxalate of 

 potass, which, when separated, is called Salts of Sorrel. In the 

 Cactacea: it exists as an insoluble oxalate of lime, in the form of 

 raphides. These bodies, which are merely needle-like crystals of 

 this salt, are very common in the vegetable kingdom. Citric Acid is 

 found in the fruits of the order Aurantiacece, as the lemon, orange, 

 lime, shaddock, &c. It is easily separated from these fruits in a 

 crystalline form. It is soluble in all its combinations with the oxides 

 of the metals, hence it does not occur as oxalic acid in the form of 

 raphides. Tartaric Acid is found in the juice of the grape. Though 

 closely resembling citric acid, it differs in forming an insoluble com- 

 pound with potass. This compound is the supertartrate of potass, or 

 cream of tartar of the shops. This salt is deposited whenever grape- 

 j uice is allowed to stand. It forms the basis of the tartar of wine 

 procured from the lees. This property of tartaric acid makes the 

 juice of the grape the most efficient compound from which to make 

 wine. The juice of fruits containing citric acid, whose salts are 

 soluble, are much less fitted for wine-making. Malic Acid is the acid 

 found in the apple, and which gives the sour taste to verjuice, as also 

 to the fermented juices of the apple and pear cider and perry. 



The chemist has described a very large number of organic acids as 

 present in plants, and every day is increasing their number. Many of 

 the colouring matters appear to be acids, which assume their par- 

 ticular colours by combination with metallic oxides, such as the 

 lecanoric, orsellic, erythric, and parellic acids, obtained from licheue, 

 used in making cudbear and archil. The vegetable alkalies, or alka- 

 loids, are also found in combination with acids ; thus, aconitiue is 

 found in combination with aconitic acid, morphia with meconic acid, 

 and a variety of others. 



The acids generally occur in combination, and sometimes supplant 

 each other. Even mineral acids will sometimes take the place of 

 organic acids ; thus sulphuric acid is sometimes found combined with 

 morphia in the place of meconic acid. On the other hand, the 

 metallic oxides will sometimes take the place of the alkaloid, and be 

 found in combination with the organic acid. In the instance however 

 of gallic and tanuic acids, there appears to be no combination with 

 alkalies or alkaloids. Tannic acid, formerly called tannine, is found 

 very generally present in the woody parts of plants. It is supposed 

 to result from the decomposition of cellulose. Theoretically, it may 

 easily be formed out of carbonic acid and water. Whether it passes 

 through the stage of cellulose is doubtful. It is of great use iu the 

 arts, especially iu tanning and dyeing, and for these purposes it is 

 obtained from the bark of oak. elm, willow, sumach, and other trees. 

 It exists in the fruits of the Chrysobalanaccd;, and the legumes which 

 are called ' divi divi.' The vegetable extracts called catechu, or cutcb, 

 and the exudations which are sold by the name of kino coni-i.it 



