xlii INTRODUCTION. 



old wood becomes more compact and harder towards the circumference than in 

 the centre. The epidermis or rind either hardens so as to prevent any increase 

 of diameter in the stem, or it distends, without increasing in thickness or split- 

 ting or casting off any outer layers. 



200. 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 expanded portion of the leaf the fibro-vascular system becomes usually very 

 much ramified, forming the smaller veins. These are surrounded and the in- 

 terstices filled up by a copious and very active cellular tissue. The majority of 

 leaves are horizontal, having a differently constructed upper and under surface. 

 The cellular stratum forming the upper surface consists of closely-set cells, 

 placed vertically, with their smallest ends next the surface, and with few or no 

 stomates in the epidermis. In the stratum forming the under surface, the cells 

 are more or less horizontal, more loosely placed, and have generally empty 

 spaces between them, with stomates in the epidermis communicating with these 

 intercellular spaces. In vertical leaves (as in a large number of Australian 

 plants) the two surfaces are nearly similar in structure. 



201. When leaves are reduced to scales, acting only as protectors of young 

 buds, or without taking any apparent part in the economy of vegetable life, 

 their structure, though still on the same plan, is more simple ; their fibro-vas- 

 cular system is less ramified, their cellular system more uniform, and there are 

 few or no stomates. 



202. Bracts and floral envelopes, when green and much developed, resemble 

 leaves in their anatomical structure, but in proportion as they are reduced to 

 scales or transformed into petals, they lose their stomates, and their systems, 

 both fibro-vascular and cellular, become more simple and uniform, or more 

 slender and delicate. 



203. In the stamens and pistils the structure is still nearly the same. The 

 fibro-vascular system, surrounded by and intermixed with the cellular tissue, is 

 usually simple in the filaments and style, more or less ramified in the flattened 

 or expanded parts, such as the anther-cases, the walls of the ovary, or carpellary 

 leaves, etc. The pollen consists of granular cells variously shaped, marked, or 

 combined, peculiar forms being constant in the same species, or often in large 

 genera, or even Orders. The stigmatic portion of the pistil is a mass of loosely 

 cellular substance, destitute of epidermis, and usually is in communication with 

 the ovary by a channel running down the centre of the style. 



204. Tubers, fleshy thickenings of the stem or other parts of the plant, 

 succulent leaves or branches, the fleshy, woody, or bony parts of fruits, the 

 albumen, and the thick fleshy parts of embryos, consist chiefly of largely de- 

 veloped cellular tissue, replete with starch or other substances (192), deposited 

 apparently in most cases for the eventual future use of the plant or its parts 

 when recalled into activity at the approach of a new season. 



205. Hairs (171) are usually expansions or processes of the epidermis, and 

 consist of one or more cells placed end to end. When thick or hardened into 

 prickles, they still consist usually of cellular tissue only. Thorns (170) contain 

 more or less of a fibro-vascular system, according to their degree of develop- 

 ment. 



206. Glands, in the primary sense of the word (175, 1), consist usually of a 

 rather loose cellular tissue without epidermis, and often replete with resinous 

 or other substances. 



§ 3. Growth of the Organs. 



207. Boots grow in length constantly and regularly at the extremities only 

 of their fibres, in proportion as they find the requisite nutriment. They form 

 no buds containing the germ of future branches, but their fibres proceed irre- 

 gularly from any part of their surface without previous indication, and^ when 



