« • • 



3^iVlU OUTLINES OF 



(211), which usually remain active during several years, but in older stems the 

 central and older layers become hard, dense, comparatively inactive, and usually 

 deeper coloured, forming M'hat is called heart-wood or duramen, the outer, younger, 

 and usually paler-coloured living layers constituting the sapwood or alburnum. ' 

 A, the medullary rays, which form vertical plates, originating in the pith, and, ' 

 radiatmg from thence, traverse the wood and terminate in the bark. They are j 

 formed of cellular tissue, keepiug up a communication between the living portion = 

 of the centre of the stem and its outer surface. As the heart-wood is formed, the ^ 

 inner portion of the medullary rays ceases to be active, but they usually may still 

 be seen m old wootl, forming what carpenters call the diver grain. , 



5, the larTc, which lies outside the wood, within the epidermis. It is, like the 

 wood, arranged in annual concentric circles (211), of which the outer older ones 

 beconie dry and hard, forming the corky layer or outer barh, which, as it is dis- 

 tended by the thickening of the stem, either cracks or is cast off with the epidennis, 

 which IS no longer distinguishable. Within the corky layer is the cellular, or 

 green, or middle bark, formed of loose thin-walled pulpy cells containing chlorophyll 

 (19-); and which is usually the layer of the preceding season. The inliermost and 

 youngest circle, next the young wood, is the liber or inner hark, formed of long 

 tough woody tissue called bast-cells. 



. ^^^' The endogenous stem, as it grows old, is not marked by the concentric ^ 

 circles of Exogens. The wood consists of a matrix of cellular tissue irregularly j 

 traversed by vertical cords or bundles of woody and vascular tissue, which are in ' 

 connexion with the leaves. These vascular bundles change in structure and direction 

 as tbey pass down the stem, losing their vessels, they retain only their bast- or long 

 wood-ceils, usually curving outwards towards the rind. The old wood becomes more 

 compact and harder towards the circumference than in the centre. The epidermis 

 or nnd either hardens so as to prevent any increase of diameter in the stem or it dia- 

 onf; ^^*^^5>"* increasing in thickness or splitting or casting off any outer layers. . 

 ^(»U. In the Leaf, the structure of the petioles and principal ribs or veins is the 

 same as that of the young branches of which they are ramifications. In the eX- 

 panaed portion of the leaf the fibro-vascular system becomes usually very much 

 rammed, forming the smaller veins. These are surrounded and the interstices filled 

 up by a copious and very active cellular tissue. The majority of leaves are hori- 

 zontal, Having a differently constructed upper and under surface. The cellular 

 stratum forming the upper surface consists of closely set cells, placed verticaUy, 

 ^ith their sniaUest ends next the surface, and with few or no stomates in the epi- 

 oermis. in the stratum forming the under surface, the cells are more or less hori- 

 zontal, more loosely placed, and have generally empty spaces 'between them, with 

 stomates in the epidermis communicating with these intercellular spaces. In ver- 

 tical leaves (as in a large number of AustraHan plants) the two surfaces are nearly 

 similar in structure. 



i 



tk^ ^!'^ •^^'^^ ^^^ reduced to scales, acting only as protectors of young buds, 

 t„rT fi.°" ^ J^^ any apparent part in tlie economy of vegetable life, their struc- 

 W Ui?'-^i ^.1 •°'' *^V'""'' 1'^''°' " °*o^° «i°^Ple; their fibro-vascular system is 



lly tI I J "a^'^^^'' 'y^^"' "'•'^^ uniform, and there are few or no stomates. 

 loavot in t^?!^ and floral envelopes, when green and much developed, resemble 

 n?tr..lf *'^"^ *n^t«^i"ical structure, but in proportion as they are reduced to scales 



vaSr r/^ir^ P^*^^'' *^'y ^«'? *^^^^ «*°'"^t««' ^"^ ^'^^'^ ^y^tems, both fibro- 

 in In ffl^' ^^"^T'^T ^}^V^^^^^ uniform, or more slender and delicate. 

 J. '.Kr.vii!^ "'*''" """^ ?\'*'^' ^Y. st'-^cture is still nearly the same. The tibro- 

 S Tfl '^; "!^r„^i^i^L-i -*-?-^<i -.t> the ceLlar tissue, is usually 



i 1 



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1 



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ait>,,.i^ \^ +i.« ci i. 1 . s '-^"'-^"iiAcu. witii tuc ceiiuiar ussue, is usuhiv 



T^ lSJ}^^''l^ ^''^ ^^y^\^^'%^^ less ramified in the flattened or expanded^ . 

 mffn. oonli^f f ^^^l^f -cases, the walls of the ovary, or capillary leaves, etc. The 

 T>ollen consists of granul^ cells variously shaped, marked, or combined, peculiar 



being constant m the same species, or often in large genera, or even Order^ 

 stigmatic portion of the pistd is a mass of looselv PPllnin.- o^-Kofo^^^ ^^afifnte 



orms 





mng 



unm 



- - r , ^ 



i"- 



other jg^rts of the plant 



A\ 





