28 PLANT STRUCTURE AND GROWTH. [cHAP. 
posited in the form of a sfzra/ thread, and these are 
called spiral vessels (fig. 31). Another method in 
which the thickening takes place is in the form of 
rings, and such cells are known as annular vessels 
(fig. 32). In others, again, which are four or five 
sided, the new deposits take place in the form of 
thin bars across the sides, and as they thus bear a 
resemblance to the “rounds” of a ladder, they are 
termed scalariform vessels (fig. 33). 
All these tissues have a definite place in the struc- 
ture (excepting the lowest forms) of plants. No 
matter how they may be arranged 
inside a plant, the outside of it is 
always invested by a layer of flat, 
close-fitting, colourless cells, called 
the Egidermis. Dotted over this 
epidermis are a number of little 
holes, each surrounded by two kid- 
ney-shaped cells (fig. 34). These 
orifices are termed Szomates, or mouths, because they 
are used for breathing purposes. Each stomate com- 
municates with the intercellular spaces we spoke 
about just now, and through them carbonic acid gas 
is absorbed from the atmosphere. 
This is the internal structure of a plant; with its 
external characters, we suspect, most of our readers 
are tolerably familiar, yet for those who are not, we 
will give a brief account of it. 
Fig. 35 represents the ideal plant. We observe 
that it consists of a main stem, more or less branched 
at its two ends. The branches at the upper portion 
of the stem bear /eaves and jlowers,—those at the 
Fic. 34- 
