Part 192. The Ferns - Plant Life-Histories. Part 3

Fig. 366.-Fern Stems (Aspidium spp.). A, underground stem (rhizome) of A. Filix-mas with rind removed to show the net-work of fibrovascular bundles. B, one mesh of this net-work enlarged to show the branches which enter a leaf to form its framework. C, cross-section of a rhizome (A. coriaceum) slightly enlarged to show the cylindrical fibrovascular system formed of two main strands, the upper (o) smaller than the lower (n), and the finer branches of these which enter the leaves. D, the fibrovascular cylinder of the same, removed and laid out flat after splitting the lower strand (u, u) in halves, leaving the upper strand (o) in the middle unbroken, as also the finer strands (b, b, b, b) which enter leaves and roots, and the larger strands (x, x, x, x, x) which enter branches of the stem. (Sachs, Mettenius.)

In the trunk of a tree-fern (Fig. 367) the prosenchyma is particularly well-developed and shows plainly a differentiation of tissues which is characteristic of all plants higher than bryophytes. Each strand is here found to contain thick-walled woody fibers (FB) and larger cells (VS) called vessels which have thin walls variously strengthened by ridges. These vessels correspond to the "pores" found in the wood of oak and other trees we have already studied. Such strands are called fibrovascular¹ bundles, and the plants or parts containing them are said to be vascular. The ultimate branches of the framework of a leaf are often nothing but single vessels. Besides the woody and the vascular tissues, which serve mainly for conducting fluids, ferns and higher plants often develop strands or layers of hardened, thick-walled cells whose function is mainly to give strength or afford protection. Such tissue is termed sclerenchyma ² in general, or sclerotic parenchyma or prosenchyma in particular. An outer layer of the cortex as at (FL) often becomes sclerotic and thus contributes much additional strength to a columnar organ. The parenchyma of a fern-stem serves very largely for the storage of reserve food in the form of starch. From the epidermis of various parts may arise hair-like or scale-like outgrowths which serve mainly to protect organs that are very young or especially need to be covered. Whereas in multicellular plants of simpler structure it was sufficient to distinguish merely different tissues, in the higher plants the differentiation has progressed so far that tissue systems must be recognized. Thus we have a tegumentary system consisting of the epidermis and its outgrowths, a vascular system comprising the vascular bundles, and a fundamental system consisting mainly of parenchyma and including meristem, the green cells accessible to light, and the pith-like internal parts in which food is stored.

¹ Fi'bro-vas'cu-lar - L.flbra, a fiber; vasculum, a small vessel.

² Scler-en'chy-ma - Gr. skleros, hard.

Fig. 367.-Tree-Fern. Section of trunk. A wedge cut from the same, magnified to show the pith (P), a fibrovascular bundle (V F V) with its sclerenchyma (F B) and vessels (V S), and the rind (F L) and epidermis (È). (Baillon.)

The stem of an Aspidium (Fig. 170) as of nearly all our native ferns, remains mostly underground as a more or less horizontal rhizome. A considerable amount of starch stored over the winter in the fundamental tissues of this perennial organ, accounts for the rapid unfolding of the leaves in spring. Some of the leaves are entirely vegetative; other leaves bear numerous minute sporangia in clusters upon the back, each cluster being covered by a shieldlike out-growth (Figs. 170, 3-5). A peculiar part of the sporangium is a ring of thickened cells running along the back (6c), which when ripe suddenly straightens, so as to rupture the thin wall in front and eject the spores. Sporangia of this type although differing in many ways from those of the adder-tongue and its kin, are doubtless homologous with them, for students of ferns find a very complete series of intermediate forms connecting the extremes. Vegetative reproduction is sometimes accomplished in ferns by the formation on various parts of buds which fall to the ground and take root (see Fig. 368).

Fig. 368.-Fern Brood-bud (Aspidium Filix-mas) on base of leaf-stalk, 1/1. (Luerssen.)

Filicinae in general agree with the ferns described in being archegoniate, vascular plants, forming true roots and stems, and having alternate leaves upon 'which are borne sporangia that discharge their spores without elaters. The number of species is reckoned at about 3,000.