calcified parts of Halimeda become somewhat transparent when mounted in Canada balsam ; 

 they shevved an illumination of the field with crossed nicols but to a markedly slight extent, 

 much less, for example, than sections of Galaxaura\ the best effect was produced from the 

 portions of the calcified plate, which were in contact with the cell walls. The very small 

 crystals of calcium carbonate are at right angles to the lumen of the cell, close to which they 

 appear : this is shewn by the characteristic black cross in polarised light. The deposit increa^es 

 with age, so that the plate continues to increase in thickness. Thus in the young joints of H. 

 cuneata, it is 0.18 mm., in the older joints 0.5 mm. thick. These thick plates are also perforated 

 by the branches of the filaments, in the lorm of tubes. The central strand remains always uncalcified. 



From the above account it may be seen that the spaces between the different branched 

 filaments of Halimeda form in each joint a connected system, bounded externally by the 

 peripheral cells and the nodes. In a living condition this system is rilled with a liquid of 

 unknown composition. From the manner in which the secretion of calcium carbonate first makes 

 its appearance, one might suppose, especially taking into consideration the conditions in other 

 calcified algae, that these spaces were at least partially filled by swollen membranes. But after 

 both decalcification and staining of Halimeda, no gelatinous matter was observed in the calcified 

 portions and it is therefore to be supposed that the calcified plate is largely deposited as a 

 crystalline precipitate from a liquid". 



Root. This is sometimes bulbous and short, sometimes longer and cylindrical. It is 

 composed of long, branched, colourless filaments varying in thickness, and often containing 

 starch. These attach themselves to the sand on which the plants grow and a thick stout mass is 

 often formeel of over 4 cm. long and about 1 cm. thick. 



Internal structure. The thallus of Halimeda is composed of continuous branched filaments, 

 in which cross-walls are entirely absent. They form three distinct layers in each joint; i,a strand 

 of filaments, which runs up the centre of the plant from the root to the apex : 2, a network 

 of branched filaments, the subcortical layer, derived from the side branches of the central strand 

 by repeated divisions ; this layer forms the main substance of the joint: 3, a single layer of 

 „peripheral cells" l ) which are however merely the termination of the filaments of the 2 nd layer. 

 These from being closely pressed together, shew a more or less hexagonal form, when seen 

 in surface view. They vary largely in size and may be long and narrow, or short and broacl. 

 In form, they may taper gradually upwards from the subcortical layer, or they may widen suddenly 

 from a very short, narrow base. This depends to some extent on the length of surface in contact 

 and the manner of connection between the side walls of adjacent cells. This connection may be 

 brought about in one of three different ways : either the side walls are in contact to a certain 

 distance, varying in different species, and remain so after decalcification ; or they lie entirely 

 free in a chalky matrix and thus fall apart on decalcification ; or they lie in a cellulose matrix, 

 rendered visible on decalcification, and always remain connected. The manner of connection is 

 not invariably constant in a species. 



The diameter of all the filaments diminishes gradually as they branch and pass to the 

 outer surface. 



1) See note ou p. 2. 



