126 
MORPHOLOGY OF TISSUES. 
becomes very thin. In this mode are produced the hollow stems of Equisetacege, 
Grasses, many species of Allium, and many exogenous plants, such as Umbelliferae, 
Dipsacus, Taraxacum, &c. Septate hollow stems have a diaphragm of firm tissue, 
traversed by fibro-vascular bundles, at the insertion of each leaf. 
In existing Cryptogams^ and in most Monocotyledons the root and every 
part of the stem retain the diameter which they had attained during their growth 
in length ; even at a great age no further increase in thickness takes place. When, 
in these plants, the stems of older specimens greatly exceed in diameter those of 
seedlings, this is a consequence of the circumstance that the apex of the stem which 
is enclosed within the leaf-bud increases in diameter as it lengthens, so that thicker 
parts of the stem are constantly emerging from the bud, until a stationary condition 
is at length reached, when the stem no longer increases in girth. When this occurs, 
as in Palms, Ferns, thick-stemmed Grasses, Aroideae, &c., the stem may attain a very 
considerable thickness while still very young, in and below the bud, without having 
any power, at a later period, to increase further in size. In the same manner the 
roots, when they first emerge from the stem, are thicker the higher they stand 
on it ; and in Pa?idanus it would appear as if roots as thick as the arm were formed 
without any secondary increase in thickness. 
Very different processes are, on the other hand, the cause of the con- 
siderable diameters of the stems and roots of Gymnosperms, Dicotyledons, and 
arborescent Liliacese. After its growth in length is completed every part is 
slender, usually only a few millimetres in thickness, rarely so much as i or 2 
centimetres ; but in the course of months and years those parts become much 
thicker ; the stem of a seedling Cistus only 2 or 3 mm. in diameter may attain, 
after two or three months, a thickness of 2 or 3 cms. ; while in the case of the oak 
the increase from the same original thickness may amount to from 40 to 60 cms. 
Since the girth increases with the age of each section, the oldest, and therefore the 
lowermost sections of the erect stem are the broadest, the successive diameters 
decreasing gradually to the summit of the tree, as is well illustrated in the slender 
conical stem of pines. In them the stem of the mature plant is a cone standing 
on its base, while in Monocotyledons and Cryptogams the cone stands on its apex, 
as is well seen in the large climbing Aroideae. In Dicotyledons and Gymnosperms 
every part of the stem is at first slender, always becoming thicker at a later period ; 
in Monocotyledons and Cryptogams, on the contrary, every part retains the thick- 
ness which it had acquired at the close of its growth in length ; the increase 
in thickness of the entire stem takes place at the upper end from the growth of 
the bud. 
This increase in girth which commences only after the close of the growth of 
length of the organ, and which then generally lasts during the whole of its life, 
may be termed, in contradistinction to that arising from growth of the bud, the 
secondary increase in thickness. It is always the resuk of an inner layer of tissue 
remaining in a condition capable of division (merismatic), and continually producing 
^ In the Lycopodiaceae of the Carboniferous period Williamson has recently demonstrated 
a secondary increase in thickness. An indication of this is found in the stem of Isoefes. (See 
Book II.) 
