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VEGETABLE PHYSIOLOGY.^No. III. 
In an article at p. 57-8 of the present volume, allusion was made to Mr. 
Main's hypothesis of the Vital membrane or Indusium ; the view taken of it was 
favourable, as it appeared more philosophical to consider the annual formations of 
sap wood and liber to be developmsnts of parts already existing, than as new 
creations. In order to do the subject all the justice within our power, and to afford 
the physiological reader matter for reflection and experiment, we feel impelled to 
offer a few additional observations which have been suggested by facts that have 
come under notice since the article alluded to appeared in print : however, before 
we do this, we must refer to Mr. Main's work in further illustration of his views. 
He observes : — 
" There are two states or degrees of vegetable life. The first is always present 
in those members which are capable of amplification, or are in the act of accretion, 
i. e. expanding from a small to a large volume. The second is that state in which 
it is only conservative, but without the power of a further growth of the members 
preserved by it. The first it is deemed proper to designate by the name of vital 
envelope, yvhence proceeds every new member of trees, shrubs, and many herbaceous 
plants. The second is, that state of the bark and alburnum which, having but 
recently come into full form and magnitude, serve as conductors of the fluids of the 
system for a certain time, but from which the actual life has for ever fled. 
" Where then does the living principle reside ? In the pith ? No. In the wood, 
or in the bark ? No, in neither of these ; but it is always found, at all times, 
between the liher and the alburnum; slightly attached to both, but united to neither : 
it is reasonable therefore to conclude that it is a distinct member of the system. 
This slender body of vitality, or vital envelope, is constitutionally compound, not 
simple, as such a thin tissue may be supposed to be, containing the rudiments of 
both roots and buds ; and moreover it is the source of all accretion, whether as to 
the magnitude, or number of the parts produced." 
In the foregoing passage we perceive the leading principle of the hypothesis : 
it presumes that every part and member of a plant are formed and coexist v/ith the 
rudimental germs of its existence ; the absorption of food and the laboration of the 
juices being processes of nutrition only, while the development and growth of all 
the members are those of accretion and enlargement, not of organization. 
We are far from disputing the existence of an indusium or vital envelope, which 
we believe is considered to be identical with the cambium of physiologists ; but 
the fact to be established or disproved is simply this : — it may be stated in the 
form of a question. Is the cambium or laborated sap capable of organization? 
or, with Mr. Main, must we suppose that the vital envelope is constructed of an 
indefinite number of distinct concentric layers, two of which are developed annually 
and become nourished or enlarged ? 
VOL. VI. — NO. LXVII. X 
