56 
HISTOLOGY OF VEGETABLES. 
with the other examples already mentioned, exhibits a 
beautiful series of colours under polarized light. 
In the cuticle of the petals of a Mallow Malva 
sylvestris, nearly every cell contains a stellate mass of 
calcareous matter, but as the masses are all crystalline, 
they are probably only raphides. 
While noticing silica in plants it will be well to make 
mention of the great abundance of this substance in 
the beds of our ponds, streams, and rivers, and in the 
waters of the ocean, formed by the accumulated remains 
of countless myriads of organized beings ; whether these 
are animal or vegetable is still a matter of contention 
between botanists and zoologists ; the former classing 
them with the Alg<z, the latter styling them Infusoria. 
They may, however, be regarded as cells, having their 
walls strengthened by a coating of silica, which like that 
of the cuticles before mentioned, is probably a secretion, 
and indestructible both by fire and acids. 
According to botanists, the Algce are divided into five 
natural orders, viz. : Diatomacece , Confervacece , Fuca- 
ce<z, Ceramiacece, and Characece. Of these the Diato- 
macecB are characterised as “ angular fragmentary bodies, 
brittle, and multiplying by spontaneous separation.” 
This order is divided into three genera, viz : Cymbellece , 
Hydrolinecs and Desmidiece ; the two first have a 
siliceous, and the last a horny skeleton. All are 
remarkable for their beauty of form, as well as for the 
markings on their surfaces, which are so delicate in 
many cases, as not only to be employed as a test of the 
