136 



KNOWLEDGE 



[July 1, 1892. 



Fig. i shows a cross section of a young elm stem. We 

 see that outside there is an epidermis (epi.) whcse 

 outer walls are covered by a layer impervious to water, and 

 called the cuticle ; a number of vascular bundles form a zone 

 about midway between the centre and the periphery. In 

 the centre is the pith, and outside the bundle zone is the 

 cortex. The vascular bundles are essentially the same in 

 structure as those of the root. The cortex is made up of 

 large cells, the outer of which are filled with chlorophyll ca- 

 green colouring matter. Outside each vascular bundle 

 is a patch of tissue formed of elongated hard-walled cells 

 destitute of protoplasm, and called sdercnchynui [fcl.) 

 (a-KXrifOi, hard). Vniting the pith to the cortex, and cut 

 by the cambium, run the medullary rays (meJ. rai/s). 

 The xylem or wood lies next the pith, and the phloem 

 {})h.) next the cortex. The phloem is sometimes called 

 the soft bast or inner bark. The stem of a cabbage, left 

 exposed for some time to the action of the elements, does 

 not decay uniformly. The cortex and pith disappear and 

 leave a framework composed of the vascular bundles. These 

 are continuous with the bundles of the root. 



The leaves are expansions of the stem. The green leaves, 

 with which we are specially concerned at present, are of very 

 varying forms, but the anatonuj of the foliage leaf of the elm 

 and of any other dicotyledon is practically the same. A 

 continuous layer of cells surrounds the whole structure. 

 This is called the epidermis (Fig. 5, ep.) ; it is covered by 



a continuous corky 

 layer called the 

 cuticle, just as in 

 the epidermis of the 

 stem. Towards 

 the upper surface 

 there are two or 

 three layers of par- 

 enchyma which 

 generally appear 

 like a palisade (Fig. 

 5 p.p), and have 

 hence received the 

 name of palisade 

 parenchyma. Below 

 that lies a loose 

 parenchyma tissue, 

 the mesophyll, Fig. 

 6, me. (jMsaoq, the 

 middle, and ^uXXov, a leaf), with many spaces between 

 the cells — inter-cellular spaces — and towards the lower 

 epidermis there is a layer of roundish cells. The con- 

 tinuous epidermis of the stem and leaf is broken by 

 numerous minute apertures — stomata [a-rwii.a., a mouth) — 

 whose structure and function were described in a previous 

 paper in Knowledge for November, 1890, on the " Breath- 

 ing Organs of Plants." There it was stated that these little 

 mouths are not breathing organs, but are the structures 

 which regulate the giving off of water by the plant ; they 

 are the trampirini/ organs. 



A section of a leaf taken through a vein at once shows 

 that this is merely ^ vascular bundle. Its course can 

 easily be traced by dissection from the leaf blade, down 

 the leaf stalk or petiole, to the bundles of the stem. The 

 veins are the ultimate ramifications of these organs, but 

 differ from them in not containing cambium."' The 

 xylem is in consequence at the upper surface and the 

 phloem towards the under surface of the leaf blade. A 



* In a monocotyledon, such as a palm or grass, tlie vascular 

 bundles of the stem are not arranged in the manner stated, bxxt are 

 disposed irregularly -. moreoTer. they are efosed biuidles, that is, they 

 possess no cambium. 



St 

 Fig. ."). — Piagrammatie transverse Section 

 of Leaf, ep., epidermis ; sch, sclerenchyma ; 

 p.p.. palisade parenchyma ; xi/., xylem ; 

 ph., phloem ; me., mesophyll ; st„ stoma. The 

 black dot in the centre of the sclerenchyma 

 cells represents the cavity of the cells. 



fckrcnchymatous sheath {scl.) more or less completely 

 sui rounds each bundle, adding strength to the structure. 

 The mesophyll and palisade parenchyma are packed with 

 chlorophyll granules. With the exception of the guard 

 cells of the stomata there is no chlorophyll in the epidermis. 

 We close this paper by alluding to a few interesting facts 

 in connection with the structure of this plant, which we 

 will not have occasion to speak of afterwards. In the 

 root we saw the pericycle, a layer of tissue which surroimds 

 the vascular bundle. It has a very definite work to perform. 

 "When a young root is to be developed the cells of the 

 pericycle situated opposite a xylem tract become active, 

 divide up and give origin to the young root which breaks 

 its way through the outer tissues of the parent root. The 

 cambium of the vascular bundle of the stem and root 

 becomes active in spring, and continues so during summer 

 and early autumn ; its cells divide up to form on the out- 

 side phloem and on the inside wood or xylem. In a two 

 or three year old twig of the elm or other tree we find a 

 mass of tfodd in the centre, and in the spring we can easily 

 strip off the bark. The juicy portion which immediately 

 surrounds the wood left is the ruptured cambium, whose 

 delicate texture enabled the operation to be performed so 

 easily. The wood increases much in excess of the phloem. 

 This can be seen by examining tlie stump of a recently 

 felled tree, such as the elm or oak. The pith has not a 

 long existence ; it soon becomes obliterated by the growth 

 of the xylem. In the hemlock, and generally in the 

 Umbelliferfe, the group of plants to which it belongs, the 

 pith breaks down and the centre of the stem is occupied 

 by a large cavity, interrupted at the nodes or parts where 

 the leaves come ofi'. The epidermis of a forest tree is 

 soon shed, and its place is taken by a corky tissue (Fig. 

 4), formed from a layer of cells in proximity to the 

 epidermis. This in turn is thrown off, and is replaced by 

 a cork layer formed in a similar manner. 



SOME FACTS ABOUT EXPLOSIVES. 



By Vaitcjhan Cornish, B.Sc, F.C.S. 



THE general phenomena of explosion are well 

 known : noise, shock, resistance otFered and 

 overcome, and, in general, some work of destruc- 

 tion wrought, such as demolition of a structure 

 or the disruption of a rock. 

 Various materials, solid and liquid, are employed for 

 the purpose of producing and utilizing the effects of ex- 

 plosion. Every such material is termed an e.rplosiir, 

 although sometimes, as in the case of gunpowder, the 

 material is in fact a mixture of several substances ; coal 

 gas and fire-damp are not called explosives, although on 

 mixing with air they are capable of exploding. An ex- 

 plosive proper contains its]own supply of oxygen. 



Explosives are of two principal kinds, of w-hich gun- 

 powder and gun-cotton are typical examples. Gunpowder 

 is a mixture of three different chemical substances, 

 whereas gun-cotton (and similarly theothernitro-explosives, 

 as, (■.;)., nitro-glycerin) is a single chemical substance, 

 composed, however, of several elements, one of these 

 elements being oxygen. 



In gunpowcler we very intimately mix together two sub- 

 stances, charcoal and sulphur, which are capable of com- 

 bining chemically with the oxygen contained in the third 

 substance, nitre. All that is necessary to bring about this 

 chemical change is, firstly, that the particles of the various 

 substances shall be brought very close together, which is 

 effected by the careful incorporation of the ingredients ; 

 and, secondly, that the temperature should be high. At a 



