ON THE PHYSICAL STRUCTURE OF PLANTS. 
43 
ON THE PHYSICAL STRUCTURE OF PLANTS. 
The physical frame of vegetables assumes very many, different cha¬ 
racters as to consistence, structure, and durability. In its simplest 
form it is composed of a tissue of small cells, united together by mutual 
attraction, or, as it would appear, by a portion of mucus with which 
their sides are covered. This mucus is dissolvable in water, because, 
when thrown into it, the cells separate from each other, and float 
singly. The arrangement of the cells is different in different plants, or 
to the form or function of the organ they compose. These organs are 
either fibres, or delicate webs, or tubes, all of which are variously placed, 
either vertically, obliquely, or horizontally. 
Fibres in their simplest state, as seen by the assistance of a micro¬ 
scope, are formed of elongated cells, united by their ends overlapping 
each other as in fig. a : — 
o 
These are sometimes single, but frequently are arranged in bundles, 
where greater solidity or strength are required. Sometimes fibres are 
disposed spirally, involving an empty centre, in which state they are 
called spiral vessels, and are found in almost all young shoots. 
Fibres give tenacity to the membrane, and form the longitudinal 
grain and transverse strength of timber ; they are elongated by the 
development of the cells of which their solid points are composed, fig. a, 
or by'unrolling the spiral coils in which they are depressed, fig. c. In 
their more or less aggregated state, they form the tough laxity of the 
petioles, the substance of the midrib, and all the ramifications of what 
is called the nerving or veining of leaves. 
Webs .—The cells are also expansible into thin tissues, by lateral 
attachment and mutual insertion of their pointed ends, fig. d. In this 
state it is seen as the cuticle of stems, leaves, and flowers, or like those 
transparent films which invest the coats of the onion bulb. Sometimes 
