THE GENUS EQUISETUM 



velopment proceeds on the whole basipetally, although some young stages also remain 

 at the extreme apex. This is a very different sequence of events from that followed by the 

 leaves of a vegetative bud, and is perhaps an additional reason for thinking that the 

 'sporangiophores' may not be foliar in nature. Since even in the tiny cones of £. scir- 

 poides a considerable number of sporangia are contained, it is usual to find every stage 

 of meiosis in one cone, and quite a range of stages in one sporangium. The reason for 

 this is that the mother cells do not form a uniform tissue but are separated into compact 

 pockets of a few mother cells surrounded by plasmodial tapetum. Every cell of a pocket 

 will be at the same stage, but adjacent pockets may be quite widely separated both in 

 space and in stage of development. This may perhaps be seen in Figs. 5^ and 217^. 



All species of Horsetail respond to normal methods of modern fixation excellently and 

 give very beautiful preparations, as the photographs in the pages which follow will per- 

 haps testify. This is a very fortunate circumstance, because even with this advantage we 

 step, with Equisetum, into a totally different order of difficulty from anything which we 

 have so far met with in the ferns. This difficulty is reflected in the hopelessly discordant 

 results obtainable from the literature, which may be quoted more as a warning than as 

 an example. Thus Tischler (19350) lists 2n = 24 for E. limosum (Steinecke, 1932); 

 2n = c. 30 for E. arvense (Lenoir, 1926) ] 2n = c. 140 for E. palustre (Lenoir, 1932), while 

 de Beer (191 3) gave n = c. 1 15 for E. arvense, and very recently Hagerup has reported 

 (see Love and Love, 1948) 2/2 = 230 for the majority of European species. 



In my experience all these records are wrong though in differing degrees, the last two 

 being slightly too high but all the others being very much too low. The main cytological 

 difficulties in estimating chromosome number here are three — the number itself is high, 

 the chromosomes are very varied in size and shape, but, thirdly, their shapes are so 

 peculiar that at first sight of meiosis they may be very misleading and may even suggest 

 the presence of multivalent pairing, as a comparison of Figs. 2 13 ^ or 222 c with the photo- 

 graphs of polyploid Biscutella on p. 8 will perhaps demonstrate. The meaning of these 

 curious appearances is only imperfectly understood, though it seems to lie in a peculiar 

 weakness of the spiral structure in which the gyres tend to fall apart rather easily under 

 the natural stresses of the forces at work in the cell, although as may be seen from Fig. 

 228, p. 230, the spiral can be quite normal in unpaired chromosomes. This type of be- 

 haviour I have seen occasionally in otherwise normal plants of Osmunda (Manton, 1945) 

 under special metabolic conditions, and it is therefore unlikely to denote any very funda- 

 mental difference between Equisetum and other plants. The closely similar behaviour of 

 certain Lycopods may, however, perhaps denote a measure of phyletic affinity with that 

 group. 



Before discussing the numerical evidence in detail it may be well to glance through an 

 array of preparations of different stages in different species. These are contained in the 

 photographs of Figs. 213-223. Diakinesis, metaphase i and anaphase 2 in four species 

 of the subgenus Hippochaete are contained in Figs. 213-216. Precisely comparable pre- 

 parations of two species of the subgenus Eu-equisetum are contained in Fig. 222<^ and c. 

 A marked difference of chromosome size may be noted in comparing these two groups of 

 figures. This difference seems to be characteristic of the two subgenera, all the forms ex- 

 amined of Eu-equisetum, namely, E. limosum, palustre, arvense, pratense, maximum, sylvaticum 



215 



