PLATE XXIV.— COMMON HORSE-TAIL (Equisetum arvense), GREAT HORSE-TAIL (E. maximum) 



and PILLWORT (Pilularia globulifera). 



Horse-tails belong to the smallest natural order among Vascular Cryptogams, there being but a single living genus and representative 

 Equisetum. They all inhabit marshy and damp places. 



Gigantic forms existed during the Carboniferous period, such as the Calamites. 



The Vegetative structures which these plants produce are extremely dissimilar — according as they are fertile or barren. The fertile 

 shoots are formed in the spring, bear spores, have no chlorophyll, and usually do not branch. The barren shoots, on the other hand, 

 which are relatively large, are formed later in the year, have abundance of chlorophyll, and branch freely, the numerous whorls of 

 branches giving that peculiar appearance suggestive of a horse's tail The business of the barren shoot is to nourish the plant ; so 

 during summer it manufactures and stores up nutriment in the underground stem, to enable it to send up a fertile shoot early next year. 



The Underground Stem or Rhizome develops Roots at each of the nodes, and produces Buds which give rise to upright shoots. 

 These buds are sometimes curiously shaped and swollen, being distended, particularly with starch, for the rapid early growth of the 

 young shoot 



mechanical 



and this ro_ e 



silica may easily be shown by fusing a piece of the stem in the hottest part of the gas flame, when little beads of glass are produced. 



The Branches are slender green filaments, given off at the nodes, and arranged in whorls. They repeat the structure of the stem 

 in being jointed and possessing leaf-sheaths. They have this peculiarity, that although formed in the axils of leaves just like ordinary 

 buds, yet instead of growing up between the leaf and the stem they burst through the base of the leaf-sheath. 



The Leaves are the funnel-shaped sheaths investing the stem, inconspicuous in the barren shoot, more prominent and swollen in the 

 fertile one. They are produced into longer or shorter teeth ; the teeth of successive whorls not being placed above one another, but 



er The Modified Leaves of the fertile shoot form the shield-like structures of the spike. These little shield-like leaves are homologous 

 with the leaf-sheaths, each appearing as a ring of tissue round the axis; the margins, in the one case, growing out into teeth or points, 

 in the other, expanding into plates or shields. 



Figs 1 and 2. The Fertile shoot is clothed at regular intervals with leaf-sheaths, and at the apex is expanded into a club-shaped 

 head covered with little stalked discs, arranged in whorls. Immediately beneath the spike is a wavy ring, representing 

 a rudimentary leaf-sheath, just like bracts or modified leaves in the neighbourhood of a flower. 

 Fig. 3. The Barren shoot is seen to have leaf-sheaths closely embracing the stem, and whorls of branches bursting through their base. 



Fiff 4. Make a longitudinal and transverse section of the Spike. 



The modified fertile leaves of the spike are placed at right angles to it and arranged in whorls, each whorl 

 supposed to correspond to a leaf-sheath, but instead of growing applied to the stem it grows at right angles to it 

 Successive whorls of leaves are closely pressed against each other, so that the discs assume a polygonal outline. 



Fiff. 5. Fertile leaf detached and examined. 



It has a short stalk bearing its shield, from the under or inner surface of which sporangia containing spores are 

 produced. The sporangia open towards the stalk by a longitudinal slit, and the greenish powdery spores readily escape. 

 Fiff 6 Embed young spike in paraffin, and cut transverse sections. Mount in glycerine, and examine under low power. 



** ' g acn 5^1^ contains a fibro-vascular bundle, which passes from the axis of the spike, and in the shield branches 



towards the insertion of each sporangium. 



Sporangium wall formed of a single layer of cells. 



Fiff. 7. Examine portion of wall under high power. . 



Cells next to stalk with ring-like thickenings. These ringed cells burst longitudinally when the spores are npe. 

 Other cells of sporangium with spiral thickenings. 



Fiff 8 Shake out some of the green spores on a slide and gently breathe upon them. ... . . 



The Spores are round or somewhat egg-shaped bodies, averaging T fo to T £ T inch in diameter, with bright green 

 contents and spirally coiled Elaters. 



Each spore is furnished with three membranes instead of two, and the outer membrane, as the spore ripens, 

 splits up into ribbon-like strips which are the Elaters. 



These Elaters uncoil as the moisture of the breath evaporates; and, by drying and moistening in this way, a per- 

 petual motion can be kept up, which is so lively and jerky that it looks more like vital than purely physical action.. 

 Fiff 9 The germinating spore produces a Prothallus, which may either bear Antheridia or Archegonia. 



*' ' xhe Prothallus is irregularly lobed, and bears the Antheridia at the end of these lobes; while in the female 



Prothallus, the Archegonia are developed between the lobes. 

 Fig. 10. The Antherozoids are not much coiled, but very stout, and have a brush of cilia at the tapering end. They are the 

 largest in the whole vegetable kingdom. 



Fiff. 11. Early development of the Embryo. . , . , , , „ 



There is first division into two cells, the upper half representing the primary axis, the lower half representing 



the root portion. Each half is again divided into two cells, the upper two representing stem and leaf, the lower 



two forming root and foot This foot is functionally the root of the young embryo, and is temporary, while the 



root proper is for the growing plant 



Fig. 12. Vertical section of lobe of Prothallus, with young plant The young plant at this stage has its first root formed, and its 



leaf-bearing axis developing leaf-sheaths. 

 Life History Diagram —The fertile branch of Equisetum produces its spike with the sporangia containing the spores. The 

 spores carried by their outspread elaters to a damp and shady spot, begin to germinate, producing either a Male Prothallus 

 with Antheridia, or a Female Prothallus with Archegonia. The Antherozoids set free, fertilise the central cell of the Arche- 

 gonium, thus producing an Embryo which grows up into the mature plant 



CLASSIFICATION OF EQUISETUM. 

 Sub-kingdom. —Vascular Cryptogams. 

 Order. — Equisetacese. 



Upright stems, hollow and jointed. 



Leaves, small, forming sheaths. > 



Fertile leaves, in whorls, forming a spike and bearing sporangia on inner surface. 



Spores, of one kind, and furnished with Elaters. 



Prothallus generally dioecious. 



