LYCOPODIACE^:. 



[ 483 ] 



LycOPODIACE.*:. 



cellular tissue (see TISSUES, VEGETABLE), 

 which in large woody stems become ligni- 

 fied by secondary deposits. The roots have 

 also a central fibre-vascular cord, connected 

 with the central cord of the stem. The 

 structure of the little-developed tuberous 

 stem of Isoetes is very different, and ex- 

 hibits a remarkable mode of growth, form- 

 ing annual layers of woody structure (see 

 ISOETES). 



The leaves are of very simple structure ; 

 but their arrangement exhibits many curious 

 peculiarities. In PSILOTUM, one of the 

 simplest forms, where they are mere minute 

 scales on a widely bifurcated stem, they 

 are alternate ; in some Lycopodia they are 

 opposite, in others whorled. When the 

 leaves are in whorls, they vary in number, 

 not only in different species, but often in the 

 same species in different localities, or even in 

 the same plant : thus, the arrangement is 

 often different on the main stem and on the 

 branches. 



When the leaves are opposite, those 

 forming the pairs sometimes differ both in 

 dimensions and form ; in Lycopodium com- 

 planatum (fig. 434), the pairs of opposite 

 leaves cross alternately at right angles, so 

 as to form four rows up the stem ; in two 

 (opposite) rows the leaves are alike and 

 flattened laterally ; of the other two rows, 

 one consists of leaves like the two just 

 described, but flattened against the stem; 

 and the fourth row (opposite the third) of 

 minute scale-like bodies. In other cases, 

 as in Selarjinella apoda (fig. 436), the cor- 

 responding leaves of the pairs are unequal, 

 and are so arranged that the smaller lie 

 in two contiguous vertical rows, on the 

 front of the stem. In most of the Lycopo- 

 diacese the leaves are simple and almost 

 sessile ; but in Tmesipteris they have a blade 

 developed into two lobes and borne on a 

 long stalk ; and in Psilotum the short scale- 

 like leaf is also divided into two lobes and 

 supported on a petiole. The leaves of 

 Iscetes are different (see ISOETES). 



The reproduction of the Lycopodiaceae is 

 not uniform. In Lycopodium the spores 

 are alike, producing monoscious prothallia ; 

 while in SelayineUa and Isoetes, as above 

 stated, two kinds of spore exist, viz. micro- 

 spores and macrospores. It is found that 

 when both kinds of spore are sown, the 

 results of their germination are totally 

 distinct. The former small dust-like spores 

 burst their outer coat after a time ; and the 

 delicate inner membrane, which is pro- 



truded, likewise bursts subsequently and 

 discharges extremely minute eel s, in each 

 of which is developed an actively moving 

 spiral spermatozoid, like those of the FERNS. 

 This breaks out and swims about rapidly in 

 the water when seen beneath the microscope. - 

 The large macrospore exhibits no external 

 change for a period varying from a few 

 w r eeks to a few months ; but a section 

 shows that a process of cell-formation has 

 commenced in its interior, which results in 

 the production of a kind of disk of cellular 

 tissue in the upper part, beneath that por- 

 tion of the outer spore-coat which exhibits 

 the three converging ridges produced by 

 the pressure of the four spores in the 

 parent sac during their development. At 

 this period the spore appears to have three 

 coats an outer, tough, coloured coat, a 

 second coat lining this, and a third which 

 lines the second over the great cavity of the 

 spore, but at the upper part invests the 

 inside of the newly-formed disk of cellular 

 tissue, which thus lies between the second 

 and third coats. This disk of tissue is a 

 prothallium; and on its upper surface are 

 developed a number of vrcheyonia of very 

 simple structure. A cell of the substance 

 of the prothallium, taking on the function of 

 an embryo-sac, develops a free cell (embryo- 

 cell) in its interior ; and the cells between 

 this and the surface become modified and 

 part, so as to leave an intercellular canal 

 between the contiguous angles of four ad- 

 joining cells, leading down to the embryo- 

 cell, the four cells growing up from the 

 surface so as to form a kind of perforated 

 cellular papilla, something like that of the 

 archegone of the Ferns. At a certain stage 

 of this development, the outer coat of the 

 spore bursts at the converging ridges, and_ 

 the angular flaps resulting turn back and 

 expose the prothallium on the upper surface. 

 One (sometimes two, but as an irregularity) 

 of the embryo-cells is then fertilized by the 

 spiral spermatozoids produced by the micro- 

 spores, if these exist at the right stage of 

 the development in the vicinity, After 

 this, the embryonal cell undergoes multi- 

 plication, first growing down as a cellular 

 filament which breaks through into the 

 great cavity of the spore. The lower end 

 lying there then increases until it acquires 

 the form of a cellular nodule, which breaks 

 out above and exhibits on its free portion 

 the first adventitious root and the first pair 

 of leaves. The rootlet makes its way down- 

 wards into the soil ; and the leaves are 



