tween the bark and the wood 

 in the early spring. 



THE ELEMENTARY TISSUES. 



similar to the blood of animals j for that fluid is the source of all the solid parts of the 

 body. It is semi-transparent and semi-fluid in the 

 internal parts of many plants, and of young plants, and those 

 with thick leaves, more particularly. In this condition it 

 is also found in great abundance between the bark and 

 the wood of all trees in the early spring months ; and then 

 separates those parts (A, Fig. 6), so as to permit the bundles 

 of young wood to pass down from the leaves, and thus 

 enable the tree to grow. It is under these circumstances 

 that the woodman strips the bark from trees which are to be 

 cut down, since then it does not adhere to the wood. The F ig. 6.-Section of the stem of a 

 t- 4-v.' a oi-Hiatirm "WhpTi this tree, the -white line showing 

 fluid is termed cambium in this situation. the colourless cambium, or 



formative fluid is met with in the external parts of plants, formative fluid, deposited be- 

 it is still semi-transparent ; but it is then solid, as may be 

 observed by scraping the surface of a box-leaf. 



Elementary Membrane. The first step in the formation of any tissue from 

 this formative fluid is the production of a solid structureless fabric, caUed elementary 

 membrane, and a modification of that fabric termed elementary fibre. It will be ob- 

 served that these elementary parts are structureless, and are produced, apparently, by 

 inspissation or thickening of the formative fluid. The process may be grossly illustrated 

 by a reference to the manufacture of paper, in which the rag-pulp (viz., rags torn into 

 threads and soaked in water) correspond^ to the formative fluid, and the paper, which 

 is subsequently produced, to the elementary membrane. The paper thus obtained is 

 fitted for the manufacture of books, and other articles ; and, in like manner, the ele- 

 mentary membrane is the solid material out of which vegetable tissues are formed. 



Elementary membrane, then, as in Fig. 7, is 



structureless; but, theoretically, it is assumed to 



consist of a layer of rounded particles, which lie 



side by side, and leave most minute spaces between 



them. This must 



be so, when we 



reflect that all 



fluids, including 



the formative 



fluid, are made 



up of rounded 



Fig, 7. Cells of EPIDERMIS, from the 

 seed of the Gourd. 



drops, with spaces between them ; and that M r hen a 

 fluid is inspissated the drops are brought closer toge- 

 ther. Thus, whilst evident openings are no t naturally 

 met with in membrane, except as shown by Pro- 

 fessor Quekett, in the leaves of a moss called sphag- 

 num (Fig. &), it must be highly though invisibly po- 

 rous, and permit certain fluids to filter through it. 



It is at first thin and translucent, as may be 

 seen in the membrane covering the seed of the 

 gourd (Fig. 7) ; but in many cases it subsequently 

 becomes thicker and more opaque. In the struc- 

 tures of the ferns (JHices) it assumes a decidedly brown colour ; and in the elaters of 



Fig. 8. Leaf of the SPHAGNUM, showing 

 at a the natural openings through the 

 tissue. 



