STARCH. 



[ 610 ] 



STARCH. 



with a low power, in their forms, appendages, 

 pores, &e. For the compound microscope 

 they afford good material for the study of 

 development of cells in the pollen, the pol- 

 len-grains themselves, and the spiral- 

 fibrous tissue of their anthers. 



STARCH.— This substance, with the ex- 

 ception of the protoplasm, the most generally 

 diiFused of all the products met with in the 

 interior of vegetable cells, occurs in the form 

 of transparent granules, of varied size and 

 form and in varying quantity, in all classes 

 of plants but the Fungi. It has been stated 

 that it exists sometimes in a diffused or 

 formless condition, but this seems question- 

 able. All starch-grains appear when newly- 

 formed as minute spherical bodies, and very 

 many never advance beyond this stage ; but 

 a considerable proportion of the grains, in 

 all cases where the starch becomes an import- 

 ant and considerable element in the cell- 

 contents, increase in size, and acquire a more 

 or less definite form, diverging from the 

 spherical, and often characteristic of the 

 particular plant in which the grain is pro- 

 duced. The grains in a single cell mostly 

 vary very much in size, on account of their 

 different degrees of development, but the 

 full-grown characteristic grains of the same 

 species of plant agree tolerably well in size. 

 One of the most remarkable peculiarities of 

 starch is the fact that it assumes a blue 

 colour when iodine is apphed to it, which in 

 most cases affords a ready means of detect- 

 ing its presence. The smallest grains are 

 almost too minute to measure, and even their 

 determination by the application of iodine is 

 sometimes unsatisfactory; the largest grains, 

 such as those of Canna and the potato, for 

 example, attain a length of more than 1-400". 

 The starch-granule is a definitely organized 

 structure, although its existence in relation 

 to that of the cell is transitory. It consists 

 of assimilated food, deposited in a definite 

 form, insoluble in the ordinary cell-sap, 

 through a process of organization analogous 

 to that by which the development of the cell 

 itself is effected. It is connected closely 

 with the cellulose structures of the cell-wall 

 by the remarkable secondary layers found in 

 the Albumen of certain seeds, composed of 

 the substance called amy loid{V\. 39. fig.266), 

 which sometimes takes a blue colour when 

 iodine is applied to it, and like starch, is 

 ultimately dissolved and removed to furnish 

 material for development. 



The structure of the starch-granule has 

 formed the subject of much debate, which. 



I 



however, seems to have originated rather 

 through considerations relating to the deve- 

 lopment, than from a difficulty in observing 

 the complete objects. Very minute granules, J 

 as above stated, appear as solid globules, but 

 when the granules acquire appreciable dimen- 

 sions, concentric lines may be observed, more 

 or less distinctly in different cases ; which 

 lines increase in number with the increase of 

 size, in many cases, however, soon becoming 

 excentrical, from the preponderating growth 

 of one side of the granule. In freshly ex- 

 tracted granules the original centre mostly 

 appears solid or with a minute black point ; 

 but if the starch is dry, the centre appears 

 hollow, sometimes is even occupied by air, 

 and some starch-grains, as in Iris pallida, 

 florentina, &c., have a large cavity. If strong 

 alcohol is applied to fresh grains, the abs- 

 traction of water Ukewise produces a hollow 

 in the central point of growth, and in all 

 these cases, cracks not unfrequently run 

 out towards the surface. The point in ques- 

 tion, the starting-point of growth, sohd or 

 hollow as the case may be, is sometimes 

 called the hilum or the nucleus ; the former 

 term arose out of the mistaken hypothesis 

 of its being a point of attachment to the cell- 

 wall; the latter term is admissible in a general 

 sense as merely indicative of its precedence 

 in age of the general mass of the grain. It 

 is sometimes asserted that this point or 

 nucleus is a pore or funnel-shaped cavity, 

 but this is altogether a mistake, as mav be 

 readily proved by gently roasting a few starch- 

 granules of the potato on a slide, and observing 

 how the expanding air blows up the dextrine 

 into which the starch is changed, in the form 

 of a bubble or bladder. Sometimes small 

 granules occur in the potato with a large 

 cavity and thin walls. 



The lines seen in the starch-granules are 

 the boundaries of superimposed layers of its 

 substance; sometimes these are very distinct, 

 sometimes very faint ; often more distinct 

 lines appear at intervals in the series of the 

 same granule (PI. 36. fig. 21), and in these 

 cases even a thin vacancy, or in the dried 

 granules a stratum of air, seems to exist 

 between the layers. The markings have 

 been described as " folds " on the starch- 

 granules, but their dependence on the exist- 

 ence of the concentric layers is beyond 

 doubt. They are seen in the proper relative 

 positions when the granules are rolled over in 

 all directions beneath the microscope ; their 

 relative numbers and forms correspond to 

 the size and stage of development of the 



