PHYSIOLOGY 



PHYSIOLOGY 



1325 



An unfailing test for starch is a blue or blue-black 

 reaction on the addition of a weak solution of iodine. A 

 bit of starch paste, or the cut surface of a potato or 

 other starchy area, will quickly show this reaction. In 

 green leaves starch may be tested by first dissolving out 

 the chlorophyll in alcohol and then staining the leaf 

 with iodine. In the same way one examines a leaf varie- 

 gated with white. The green or colored parts of the 

 leaf alone will show starch, the white areas showing no 

 blue or purplish coloration, demonstrating that they 

 have formed no starch. 



It reqxiires a glass apparatus, such as is shown in 

 Fiir. 1784, to demonstrate that a land plant cannot form 

 starch in an atmosphere free of carbon dioxid. Over a 

 small potted plant (or better, the plant may be trans- 

 planted to a glass jar protected at the top by rubber 

 cloth) is placed the vessel a, cemented to the glass 

 plate, 6. A solution of caustic potash in c absorbs the 

 carbon dioxid in the vessel, and all air admitted must 

 pass through the U-tube d, which contains at e pumice 

 stone soaked with caustic potash. This plant exposed 

 to direct sushine for a few days will show no starch 

 formation on testing its leaves. In a short time it will 

 also become unhealthy and cease to grow. 



Not only does chlorophyll act in conjunction with sun- 

 light for the manufacture of starch; but, in general, 

 sunlight is absolutely necessary in order that chlorophyll 

 may be normally developed. Seeds germinated in a 

 darkened vessel or potato sprouts which have pushed 

 into growth in a darkened cellar will remain yellow or 

 white. Moreover, the plants will grow long and slender, 

 and death will result when the plantlet can no longer 

 draw upon the parent part for starchy matters. The 

 total dry weight of such plants will not be greater than 

 the dry weight of the original seed or tuber. In this 

 connection it might be stated that fern spores require 

 some light in order that germination may occur, while 

 the germination of ordinary flowering plants is slightly 

 retarded in the presence of light. 



The sugar into which starch is converted for translo- 

 cation is abundant in the leaves ; and it is also trans- 

 ferred to all living parts of the plant, along with other 

 organic products, besides the various salts in solution 

 which have come up to the leaves from the soil. Under 

 the influence of the active protoplasm of the leaf -cells 

 or of other tissues, more complex compounds necessary 

 in growth may be formed. All parts of the ordinary 

 plant are dependent upon the roots for a supply of the 

 mineral salts and nitrogen ; but, on the other hand, 

 they are entirely dependent upon the leaves for the first 

 organic substance, and for much prepared food. 



Growth, and the Differentiation of Structure. Plant 

 growth is apparent to the unaided eye as change in 

 form and size of organs and tissues. The real evi- 



1784. An apparatus for demonstrating that plants cannot form 

 starch in the absence of carbon dioxid. 



dence of growth is in the multiplication of the tissue 

 cells, or of constructive changes in the form and bulk 

 ' these cells. Growth may be so rapid that it may be 

 readily measured, or it may proceed so slowly or by such 

 obscure internal modifications that very little external 

 indication of the complex processes will be manifest. 



Even under such adverse conditions as that of decidu- 

 ous trees in winter, some slight growth may be taking 

 place, and it is not well to dissociate from the idea of a 

 living plant all growth phenomena. Nevertheless seeds 

 and other air-dried plant parts may live without growth 

 for considerable periods. 



The growth in size and length of different plant or- 

 gans is very various. The zone of growth in the root is 

 just back of the tip, so that if an ink mark 

 be made immediately behind the tip and 

 another a quarter of an inch further back, 

 almost the entire growth extension of the 

 root will take place within the region thus 



1785. A beech twig in winter 

 condition. 



1786. A similar twig 

 when growth has 

 begun in the 

 spring. 



marked. There is no growth in the very tip, because 

 it is hard and protected by a stout cap to aid in forcing 

 its way through the soil and around hard obstacles. The 

 region of greatest cell division is nearer the tip than the 

 region of greatest elongation. On the other hand, the 

 young stems of annual plants and the rapid elongation 

 of the young shoot may for a time show growth through- 

 out the entire extent. The winter condition of a beech 

 twig is shown in Fig. 1785, and the long, delicate, 

 overlapping scales of the buds are very evident. Each 

 bud is an incipient branch, as is readily seen in the 

 spring when the buds elongate ; the delicate scales 

 separate farther and farther from one another, each 

 bearing a little leaf in its axil, and marking a now 

 distinct joint or node in the new branch. Various 

 stages of this general elongation are evident from 

 Fig. 1786. Finally as the branch lengthens through- 

 out its entire extent, the scales drop, the leaves expand, 

 the older nodes cease to elongate, and the wave of elon- 

 gation follows a few nodes behind the terminal bud. 



In the common woody plants growth in diameter is 

 accomplished by means of a distinct but thin layer of 

 tissue functioning as the zone of cell division, or cam- 

 bium. The cambium is located just between the wood 

 and bark. In fact, it divides a complete ring of fibro- 

 vascular bundles into an inner or woody portion (xylem) 

 and an outer or bast and sieve-tube portion (phloem). 

 Each year it gives rise on the inner side to a layer of 

 wood and on the outer side to a layer of bark, thus each 

 year covering up and pushing to the center, as it were, 

 the old wood, and pushing outward the old bark as a 

 protective covering. By this process the inner wood 

 retains its former dimensions, but the bark must con- 

 stantly expand to cover the increasing diameter of the 

 tree, and so it breaks into rifts and ridges of various 

 forms, or else peels off periodically. The differentiation 

 of the tissues in different parts denotes different physi- 

 ological functions. Thus the woody part of the young 

 rings conducts the water and other soil foods, and 

 through the woody bundles of the leaf-stalk, veins, and 

 veinlets it is distributed throughout the plant body. 

 The bark or phloem portion of the bundles is largely 



