INTRODUCTION. XXXI 



formed into petals, they lose their stomates, and their systems, both flbi'o-vaseular and 

 cellular, become more simple and uniform, or more slender and delicate. 



203. In the stamens and pistils the sti'ucture is still nearly the same. The fibro- 

 vascidar system, surrounded by and intermixed with tlie cellular tissue, is usually sim- 

 ple 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. Tlie stigmatic 

 portion of the pistil is a mass of loosely cellular substance, destitu.te of epidermis, and 

 usually is in communication with the ovary by a channel rmming 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 developed cellular tissue, re- 

 plete 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 development. 



206. Grlands, 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 sub- 

 stances. 



§ 3. Groivth 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 contain- 

 ing the germ of future branches, but their fibres proceed irregularly from any part of 

 their surface without previovis indication, and when their growth has been stopped for 

 a time, either wholly by the close of the season, or partially by a deficiency of nutri- 

 ment at any particular spot, it will, on tlie return of favourable circumstances, be resumed 

 at the same point, if the growing extremities be uninjured. If during the dead season, 

 or at any other time, the growing extremity is cut ofi", dried up, or otherwise injured, or 

 stopped by a rock or other obstacle opposing its progi'ess, lateral fibres will be formed 

 on the stiU Uving portion ; thus enabling the root as a whole to diverge in any direc- 

 tion, and travel far and wide when lured on by appropriate nutriment. 



208. This growth is not however by the successive formation of terminal cells attain- 

 ing at once their fidl size. The cells first formed on a fibre commencing or renewing 

 its growth, will often dry up and form a kind of terminal cap, which is pushed on as 

 cells are formed immediately under it ; and the new cells, constituting a greater or 

 lesser portion of the ends of the fibres, remain some time in a growing state before they 

 have attained their full size. 



209. The roots of Exogens, when perennial, increase in thickness like stems by the 

 addition of concentric layers, but these are usually much less distinctly marked ; and 

 in a large number of perennial Exogens and most Endogens the roots are annual, perisli- 

 ing at the close of the season, fi-esh adventitious roots springing from the stock when 

 vegetation commences the following season, 



210. The Stem, including its branches and appendages (leaves, floral organs, etc.), 

 grows in length by additions to its extremity, but a much greater proportion of the ex- 

 tremity and branches remains in a growing and expanding state for a much longer 

 time than in the case of the root. At the close of one season, leaf-buds or seeds are 

 formed, each contauiing the germ of a branch or young plant to be produced tlie follow- 

 ing season. At a very early stage of the development of these buds or seeds, a com- 

 mencement may be found of many of the leaves it is to bear ; and before a leaf unfolds, 

 every leaflet of which it is to consist, every lobe or tooth which is to mark its margin, 

 may often be traced in miniature, and thenceforth till it attains its full size, the branch 

 grows and expands in every part. In some cases however the lower part of a branch 



