xlvi OUTLINES OF BOTANY. 
veins 1s the same as that of the young branches of which they are ramifi- 
cations. 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 interstices 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 epi- 
dermis. 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 vege- 
table life, their structure, though still on the same plan, is more simple; 
their fibro-vascular 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 va- 
riously 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 plants, 
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 
developed cellular tissue, replete with starch or other substances (192), de- 
posited 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 har- 
dened 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 development. 
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. Roots 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 
