50 
The life history of the photophyta does not present any difficulty 
to the microscopic observer ; they occur in such abundance that 
patience alone is required to trace their gi-owth from the spore to 
the perfect form. 
The question is often asked, what are diatoms ? This query, 
like many others in natural history or science, is more easily asked 
than satisfactorily answered. The inquirer might be told that they 
were a species of unicellular algae, capable of secreting silex : this 
) although perfectly correct, would not probably enlighten the 
questioner. He might also be told, in simpler language, that they 
were plants— a reply that would probably be met with by an incredu- 
lous “oh!” To those unacquainted with the simpler forms of life, 
roots, stems, and leaves are always supposed to be as necessary to 
constitute a plant, as organs for locomotion, sight, and hearing, 
and certainly a mouth and stomach, would be considered necessary 
to constitute an animal. In both cases the supposition is incor- 
rect ; the simplest forms of plants exist without leaves, stems, or 
roots, and are, nevertheless, as truly plants as those simple organ- 
isms which are destitute of feet, eyes, ears, mouths, or stomachs, are 
animals. 
In order to explain what diatoms are, I ■will, with your permis- 
sion, endeavour to describe, in as simple a manner as possible, the 
plant ceU, and if I can succeed in making this understood, I think 
you will be inclined to admit that diatoms belong to the vegetable 
kingdom. 
The vegetable cell may be looked upon as a membranous vesicle 
filled with a fluid resembling the sap fluid of larger plants. The 
walls of the cell consist of two layers ; the external portion differs 
from the internal in composition and structure. The internal cell 
is caUed the primordial utricle, or first formed ceU ; its existence is 
necessary to the vitality of the cell j it is thin and invisible when 
in contact with the external wall, but under certain changes which 
take place in the development of the cell, as by the expulsion of 
its contents, it becomes apparent. Chemical analysis shews it to 
be distinctly albuminous, in this respect resembling animal tissues. 
The simplest form of vegetable cell is well illustrated in the Tomla 
cerevisioi, commonly known as the yeast plant, and is found so 
abundantly in yeast, that it may be said to consist almost entirely 
of this minute fungus ; it is to the present time a disputed point 
