ELEMENTARY BOTANY. XXIX 



usually curving outwards towards the rind. The 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 splitting 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, form- 

 ing the smaller veins. These are surrounded and the interstices filled up by a copious 

 and very active cellular tissue. The majority of leaves are horizontal, having a diffe- 

 rently 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 ge- 

 nerally 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-vascular system is less rami- 

 fied, 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 trans- 

 formed 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 

 8I mple i n 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 

 oeing constant in the same species, or often in large genera, or even Orders. The stig- 

 matic portion of the pistil is a mass of loosely cellular substance, destitute of epidermis, 



n <-» usually is in communication with the ovary by the channel running down the centre 

 of the style. J 



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

 rtrTa 1 " branche s, the fleshy, woody, or bony parts of fruits, the albumen, and the 

 2? flesh y P art s of embryos, consist chiefly of largely developed cellular tissue, re- 

 Piete 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 

 eon * 0r more cells P laced end to end. When thick or hardened into prickles, thev still 

 vaT 1 U8Ua11 ^ of cellular tissue only. Thorns (170) contain more or less of a nbro- 



9r!ft >y 8tem ' acC0I "ding to their degree of development, 

 loo ,Y land8 > in the primary sense of the word (175, 1), consist usually of a rather 

 sbnee ti8SUe without epidermis, and often replete with resinous or other sub- 



§ 3. Growth of the Organs. 



fc?*!? ^ grow in length constantly and regularly at the extremities only of their 

 inaa m P r °P° rti on as they find the requisite nutriment. They form no buds contain- 

 er £ ertn of future branches, but their fibres proceed irregularly from any part ot 

 a tin, -^ ^tnout previous indication, and when their growth has been stopped tor 

 nWY lt,her wh <% by the close of the season, or partially by a deficiency of nutri- 

 sumti I ?"J P arti eular spot, it will, on the return of favourable circumstances, be re- 

 2* 5 the same point, if the growing extremities be uninjured. If during he dead 

 2erf° r at an y other tira e, the growing extremity is cut off, dried up, or othen . e 

 J lred ' or stopped by a rock or other obstacle opposing its progress, lateral fibres • ill 



