UTRICULAR STRUCTURES.

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The grounds for the opinion here expressed are as fol-lows : In many starch-grains a layer different from therest, and surrounding the nucleus, may be more or lessclearly perceived. In some, this shows itself as a tolerablythick membrane which iodine does not colour (fig. 14).The starch-grains are hollow; sometimes the cavity is solarge that the starch invests the membrane merely as athin layer (fig. 15). The layers become softer and con-tain more water toward the interior, and are tougher andmore solid toward the exterior; from analogy to the lig-nifying layers of cells, the softer are to be considered asthe younger, the harder the older.

Nothing is known of the origin and propagation ofthe starch-utricles, except what regards the external condi-tions under which they are produced; we know that theymay be formed free in the cell, and this either in mucilageor in chlorophyll; in utricles, namely, in the interior ofnuclear utricles (in the Fucoidese*), of prolific utricles( Caulerpa ), and of colour-utricles (pi. II, fig. 10, 11, 12,13 ’ 1? )‘

The shape in which the starch-utricles appear is pro-bably at first always spherical. They may remain spheri-cal or extend lengthways, so as to become almostcylindrical, or grow in a plane, so as to assume a tabularshape. The form of the full-grown starch-grains is ex-ceedingly varied and mostly irregular, so, however, thatgranules which originate free, are always bounded by onesingle curved surface.

The granules lie free or united in groups of from twoto twelve, and even more. A group of this kind is, notvery aptly, termed a compound grain. The individualgrains or utricles are parenchymatous ; the sides directedtowards the interior of the group are plane, those whichform the surface of it are curved (convex outwards). Allthe grains united together in a single heap have beenproduced in one utricle. They are at first spherical, butfrom the reciprocal pressure which they undergo, in con-sequence of their growth, they acquire a parenchymatous* Nageli on Cell-Formation; Fart I , Ray Trans. 1845, pi. vi, fig. 10,