UTRICULAR STRUCTURES. 185 



The grounds for the opinion here expressed are as fol- 

 lows : In many starch-grains a layer different from the 

 rest, and surrounding the nucleus, may be more or less 

 clearly perceived. In some, this shows itself as a tolerably 

 thick membrane which iodine does not colour (fig. 14). 

 The starch-grains are hollow ; sometimes the cavity is so 

 large that the starch invests the membrane merely as a 

 thin layer (fig. 15). The layers become softer and con- 

 tain more water toward the interior, and are tougher and 

 more solid toward the exterior ; from analogy to the lig- 

 nifying layers of cells, the softer are to be considered as 

 the younger, the harder the older. 



Nothing is known of the origin and propagation of 

 the starch-utricles, except what regards the external condi- 

 tions under which they are produced ; we know that they 

 may be formed free in the cell, and this either in mucilage 

 or in chlorophyll ; in utricles, namely, in the interior of 

 nuclear utricles (in the Fucoideas*), of prolific utricles 

 (Caulerpd), and of colour-utricles (pi. II, fig. 10, 11, 12, 

 13, 17). 



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 almost 

 cylindrical, or grow in a plane, so as to assume a tabular 

 shape. The form of the full-grown starch-grains is ex- 

 ceedingly varied and mostly irregular, so, however, that 

 granules which originate free, are always bounded by one 

 single curved surface. 



The granules lie free or united in groups of from two 

 to twelve, and even more. A group of this kind is, not 

 very aptly, termed a compound grain. The individual 

 grains or utricles are parenchymatous ; the sides directed 

 towards the interior of the group are plane, those which 

 form the surface of it are curved (convex outwards). All 

 the grains united together in a single heap have been 

 produced in one utricle. They are at first spherical, but 

 from the reciprocal pressure which they undergo, in con- 

 sequence of their growth, they acquire a parenchymatous 



* Nageli on Cell-Formation ; Part I, llay Trans. 1845, pi. vi, fig. Ifi. 



