418 SYSTEMATIC BOTANY. 



are furnished with filiform processes, called elaters (fig. 494, /), consisting 

 of two short threads attached at one side, coiled spirally round the spore 

 before it is mature, and unwinding with elasticity when the spore is 

 discharged from the sporange. The erect stems die down annually, 

 while the rhizome persists. 



The asexually produced spore of the Equisetaceae contains a nucleus and 

 chlorophyll granules, and it is, perhaps, owing to this high state of internal 

 organization that its powers of germination are retained for only a few 

 days at the most. The first sign of germination is an enlargement 

 of the spore and the assumption of a pear-like form, during which it 

 divides into two cells; the smaller, possessing almost colourless con- 

 tents, grows out in the form of a long root-hair, and the other, contain- 

 ing the chlorophyll of the spore, by repeated cell-division ultimately pro- 

 duces the prothallium. The prothallia of the Equisetacea3 are small, flat, 

 thalloid (as their name implies), chlorophyll-containing bodies consisting, 

 in some parts, of several layers of cells, and supplied with irregular arm- 

 like lobes. On the prothallia, which are usually dioecious, are to be found 

 the sexual organs antheridia or archegonia as the case may be. The 

 male prothallia are usually a few millimetres long, and the female often 

 as much as half an inch ; but, although individual in the minor details of 

 form r different species are distinguished by differences in breadth, length, 

 and the nature of their branching, The antheridia, or male organs, are 

 produced at the end of the large and between two smaller secondary lobes 

 of the male prothallia. They contain upwards of 100 large spermatozoids 

 (the largest produced by any Cryptogam), which on being set free swim 

 about in the water, without the presence of which they cannot be emitted. 

 The archegonia, or female organs, arise at the base of the lobes of the 

 female prothallia, and consist each of a few cells so arranged as to form a 

 canal leading to an embryonic cell in the centre of the other cells of the 

 archegonium. The central embryonic cell is fertilized by a spermatozoid 

 through the conductive agency of the canal. Immediately after fertiliza- 

 tion the canal cells close, and the embryonic cell begins to increase in size, 

 and the cells of the neighbouring tissue undergo a corresponding increase 

 in number. By-and-by the embryonic cell also divides; and'after the 

 division has been often repeated, we begin to see differentiation in the cells, 

 which is the result of this process. The growth proceeds by an apical 

 cell, and a leaf-bearing shoot with a rhizome and root is soon to be seen 

 and recognized as a young Equisetum as it is commonly to be found. 

 When the Equisetum has attained maturity, the asexual spores (from 

 which we started) are again borne on the erect metamorphosed stem, and 

 so the life-history proceeds in the alternating sexual and asexual gene- 

 rations. 



Affinities, &c. The well-known fossil Calamites resemble in a very striking 

 degree the existing representative of Equisetum, not only in the tissues of 

 the vegetative body, but even in the possession of elaters in the fructifi- 

 cation. The plants of this Order at present existing belong all to a single 

 genus, which is very unlike any other form of Cryptcgamous plants. In 

 external appearance the stems have no little resemblance to those of 

 Ephedra and Casuarina ; but their internal organization is totally different. 

 They resemble the Grasses in having a deposit of silex in the epidermal 

 tissues of fistular erect stems, in E.hyemale so abundant that the ashes of 



