EQUrSETACEM. 



373 



peltate scale. The very thin-walled cells of the wall have previously formed spiral 

 thickening-ridges on the dorsal, annular ones on the ventral side of the sporangium, 

 arising, according to Duval-Jouve, in the case of J^. limosinn, with extraordinary 

 rapidity immediately before the dehiscence. The development of the spores of 

 Equisetum, after they have made their appearance as naked primordial cells by 

 the division into four of their mother-cells, shows the pecuharity of a successive 

 formation of distinct coats. Each spore forms first of all an outer non-cuticularised 

 coat capable of swelling, which, splitting subsequently into two spiral bands, forms 

 the so-called Elaters, a second and third coat soon afterwards making their appear- 

 ance within it. All three lie at first closely one upon another like successive layers 

 of a single coat; but when the spore is placed in water, the outer one, even at 

 this period, swells up strongly and becomes detached from the others (Fig. 280, 

 B). The three coats may be easily distinguished even in the quite fresh spore 

 when placed in distilled water (/!), (in the case of E. limosum), the outer one 

 (i) being colourless, the second (2) light blue, and the third (3) yellowish. As 



riG. 280.— Development of the spores of l-.qnisetum limosuin (x8oo) ; A unripe spore with three coats just placed 

 in water ; B the same after two or three minutes in water, the outer coat having become separated, a large vacuole 

 is seen by the side of the nucleus ; C conunencement of the formation of the elaters on the outer coat c (= i in Figs. 

 A and B) ; D, H the same stage of development in optical section after lying twelve hours in glycerine, e the outer 

 coat ; 2, 3, the inner coats separated from one another ; F the outer coat split into spiral elaters, coloured a beautiful 

 blue by Schultz's solution. 



the development advances, the outer coat is separated like a loose investment from 

 the body of the spore (C, d, e), and at the same time its division into elaters is 

 first indicated. The optical longitudinal section shows that the spiral thickening- 

 bands of this coat are separated only by very narrow spaces of thin membrane [D, 

 E) ; these at length entirely disappear, and, when the surrounding air is dry, the 

 thicker parts separate from one another as spiral bands, forming when unrolled 

 a four-armed cross; they are united by their centre, and attached there to the 

 second coat. It is probably this spot which may be recognized even in the 

 unripe spore 'in the form of an umbilical thickening {11 in A and E). In the 

 fully developed elaters an external very thin cuticularised layer may be distin- 

 guished. They are extremely hygroscopic ; when the air is damp they are rolled 

 round the spore, but when dry are again unrolled. When this alternation takes 

 place rapidly (as when lightly breathed on under the microscope), the spores are set 

 in active motion by the bendings of the elaters. If spores, the outer coat of which 



