FOEMATION OF STARCH-GRAINS 449 



bulbs, coriiis, fleshy roots, and other similar structures are per- 

 haps the most easily examiDed. In seeds the reserve materials 

 are deposited for the support of the youni,^ embryo, which will 

 need them at the onset of germination. Tubers will require 

 them for the support of the young shoots to which they give 

 rise. Fleshy roots have to support a vigorous outgrowth after 

 the expiration of winter. In the axes of woody plants we find 

 stores laid up in sheaths of cells, such as the endodermis or 

 peric,ycle, or in the medullary rays existing in the secondary w^ood. 



In these reservoirs the stored substances are deposited 

 in an insoluble form, or at any rate in a shape which is not 

 suitable for rapid transport. In most plants the reserve carbo- 

 hydrates are stored in the form of starch grains, which gene- 

 rally differ from those in the chloroplastids by their larger size 

 and apparent complexity of composition. They are formed at 

 the expense of the sugar of transport, usually by special bodies 

 known as Jeucoplasts from their white colour. These bodies 

 are very nnich like chloroplasts, except that they contain no 

 chlorophyll. The leucoplast appears to absorb the sugar and 

 pour out starch from some part of its surface. Layer after 

 layer is so excreted, till a recognisable starch grain is produced, 

 which shows the manner of its growth b}^ faint striation of its 

 surface, the striae showing the limits of the successive lamellae 

 laid down. In some cases the lamellae are strictly concentric ; 

 in others, as in the potato, the end next the leucoplast gradually 

 becomes broader and broader, so that the shape somewhat 

 resembles that of an oyster- shell. Fig. 1187 shows a group of 

 leucoplasts actively forming starch grains. Fig. 1188 shows the 

 striations of the complete grain. 



Some grains often found in the potato are not so simple in 

 their structure. These are represented in ^z^s. 1189 a and b. The 

 former usually arise by two or more originating in the interior of 

 the leucoplast ; as they grow they become closely pressed to- 

 gether and constitute a compound grain. Fig. 1189 b. shows what 

 is often called a semi-compound grain. In such a formation the 

 leucoplast produces two grains on opposite sides, and as they 

 increase they come into contact. The leucoplast is so reduced 

 to a ring surrounding them at their point of union, and its con- 

 tinued activity then forms new layers surrounding the whole. 

 The leucoplast is gradually used up in its activity and disappears, 

 leaving the grains of starch free. Many variations of its 

 behaviour have been noticed in different cases 



Besides this formation of large starch grains by leucoplasts 



VOL. II. G G 



