20 



KNOWLEDGE 



[January 1, 1896. 



grows chiefly on the apple, pear, service tree, hawthorn, 

 and occasionally on limes, poplars, willows, and lir.s. 

 The accompany in- illustration (Fig. 1) is from a plioto- 



FiG. 1. — Mistletoe growing on a Crabtree. 



graph, kindly provided hy Mr. Geo. Paxtou, of a plant 

 grovring on a Siberian crabtree. This mistletoe is a 

 very large one, being five feet three inches in height and 

 fourteen feet six inches in circumference. 



The young mistletoe, while still enclosed by the seed 

 coat, contains the green colouring matter chlorophyll. This 

 is an unusual circumstance, as chlorophyll is developed, as 

 a rule, only in tissues exposed to sunlight ; and equally 

 anomalous is its presence in the cells of the nourishing 

 matter surrounding the embryo. When the seed germinates, 

 the part of it below the cotyledons, 

 the radicle as it is termed, emerges 

 first from the seed coat. It is im- 

 mediately brought into contact 

 with the bark of its host. " This 

 is the case even when the seed 

 chances to stick with the radicle 

 of the seedling pointing away from 

 the branch ; the whole axis of the 

 embryo curving towards the surface 

 of the bark in a very striking 

 manner." The radicle then 

 presses itself against the tree, 

 and at the same time becomes 

 disc-shaped. Then from its 

 centre a growth arises which 

 pierces the stem of the host and penetrates as far as the 

 wood. This portion is called a sinker, borer, or haustorium. 



Fig. 2.— T rans V 1- : 

 section of Tliorn Acacia 

 with Mistletoe. 



It is shown in longitudinal section in Fig. 2, which is a 

 transverse section stem of the " thorn acacia " on which a 

 mistletoe plant has fastened itself. 



Let us now glance brielly at the structure and functions 

 of the component parts of the stem of a green plant that 

 yearly increases in thickness, in order that we may be in a 

 position to appreciate the relations existing between the 

 parasite and its host, ^^'o shall examine the transverse 

 section of the stem of the mistletoe itself, an illustration of 

 a portion of which is given in the accompanying sketch 

 (Fig. 3). The central part of the stem is at first occupied 

 by a cylinder of pith (ji), 

 whose thin-walled cells, as a 

 rule, become obliterated later 

 on. Surrounding it is a zone 

 of wood (.n/), composed of 

 cells and tracheic. The cells 

 contain protoplasm, and so 

 are capable of further growth. 

 The trachere are devoid of 

 protoplasm. The cells act as 

 storehouses of starch, while 

 the trachetB are the rapid 

 carriers of " crude " sap from 

 the roots to the leaves. 

 Outside the wood is a layer 

 or layers of cells, to whose 

 activity is due the increase 

 of the stem's girth. It is 

 termed the cambium (en). 

 The cambium is enclosed by 

 the phloem (pJi). In the 

 mistletoe it appears in wedge- 

 shaped patches in cross- 

 section. Its constituents are 

 ordinary cells containing 

 protoplasm, and sieve-tubes ; 

 the latter are cells whose 

 horizontal, and sometimes 

 also lateral, walls are pierced 

 by holes, giving them the 

 appearance of sieves. The 

 phloem conducts elaborated 

 food-material from the leaves 

 to wherever growth is going 

 on. Much of this food is of 

 a colloid nature, and so it is 

 necessary to provide aper- 

 tures to enable it to pass 

 rapidly from one place to another. The xylem, phloem, and 

 cambium are almost invariably associated the one with the 

 other, and in young annual stems, such as the Common 

 Sunflower, or in the leaf-stalks of the Ehubarb, form the 

 stringy fibres so noticeable when these structures are 

 broken across. These strings are known as the vascular 

 bundles, and as fibrous strengthening material is often 

 associated with them (for purposes of support) they have 

 received the name of fibro-vascular bundles. 



By the division and sub-division of the cambium cells 

 yearly, zones of wood and phloem are produced ; but the 

 wood increases disproportionately to the phloem, and ulti- 

 mately forms the main portion of the stem. To the 

 tissues encircling the vascular bundles the name cortex (c) 

 is applied. The outer cortical cells contain chlorophyll, 

 and so are active in carbon assimilation. The epidermis 

 (ep) is the layer of cells enclosing the cortex ; its outer 

 walls are corky, and so prevent the evaporation of slow 

 currents of water passing fi'om the roots to the 

 leaves. When the epidermis becomes ruptured during 



Fio. 3. — Transverse section of 

 Stem of Mistletoe : ep, epidermis ; 

 c, cortex ; ph, phloem ; ca, cam- 

 bimn; x^, xylem (wood) ; p, pith. 



