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UTRICULAU structures.

and not homogeneous granules, follows from this :—1st,that when large enough, a membrane may be detectedon them which is not coloured by iodine, while the con-tents become brown (pi. II, fig. 18); and 2d, that themucilaginous contents may become frothy and hollow.The membrane becomes especially manifest, when thecontracting contents separate in places from the mem-brane, in utricles which come in contact with water ortincture of iodine (fig. 18, b, c ).

Little can be observed of the origin of the mucilage-utricles. They first appear as very minute globules ofmucilage. It is possible that a small quantity of muci-lage becomes agglomerated, individualized, and acquiresa membranous covering. I have not observed a propaga-tion of the mucilage-utricles.

The shape of the mucilage-utricles is spherical fromthe beginning, and it always remains so. The growthserves merely to expand the utricles uniformly.

The contents are originally homogeneous. They remainin this condition, or one or more hollow spaces appear inthem.

The membrane is delicate, and seldom attains any con-siderable thickness. In Chara and Nitella only have Iyet seen the mucilage-utricles with tougher membranes.The membrane is also thin here originally, and either in-visible or only indistinct. By the application of iodinewe may mostly succeed in seeing it more clearly. It thenappears distinctly bordered by two lines (Big. 18, a). Ata later period minute points (b) appear on the outersurface, which with the increase of size show themselvesto be little spines (c, e). In old utricles the spines dis-appear, and the outer surface of the membrane is irregularand uneven (d). During this the membrane has beenconstantly increasing in thickness.

Looking for an analogy for the spines just described,one might think of the cilia of the germ-cells of Vaucheria,and notice that the mucilage-utricles of the Characeselikewise move (rotate), while the mucilage-utricles of other