360 LYCOPODIALES 



pared with those seen in Isoetes. Here, as in L. Selago and L. Phlegmaria, 

 the hypobasal tier forms the suctorial organ only, and takes no direct 

 part in the establishment of the plant. The epibasal tier is like that of 

 L. Selago as regards the parts which it initiates and in the positions 

 which they severally hold, but differs in its growth in length being 

 stunted, and in the early ascendancy of the cotyledon, which condition it 

 shares, however, in some measure with L. Phlegmaria : it differs also in 

 the late definition of the apex. But the position of the latter relatively 

 to the whole embryo is the same, for the stem originates in close 

 relation to the centre of the upper tier of the embryo, as it does in 

 all the Lycopods where the embryogeny has been exactly followed. The 

 apical cone is small in bulk and late in appearance, these being probably 

 correlative consequences of the early advance of the cotyledon. It is 

 thus possible to see even in the embryo of Isoetes some clear relation 

 to the plan which, with such curious modifications, underlies the embryo- 

 geny of the Lycopods. 



i 



We are now in a position to enunciate a comparative view of the 

 embryogeny as known in the Lycopodiales, and to state it so as to place 

 the several curiously divergent types in what is believed to be their 

 natural relation to a probable primitive embryogeny. In so doing pro- 

 minence is given to the more constant features, while only a subsidiary 

 place is given to those characters which are less stable. 



In those Lycopods in which the embryogeny has been exactly followed, 

 the embryo consists of a suspensor and two tiers of four cells each 

 composing the embryonic body : the two tiers are separated by a wall 

 which may be called the "basal" wall (Fig. 182 b, /;). This seems to be 

 a general condition, subject only to minor modifications : in Isoetes, 

 however, the suspensor is entirely wanting. In the very various develop- 

 ments which follow, the most constant feature is undoubtedly the close 

 relation of the stem-apex to the point of intersection of the octant-walls 

 in the epibasal tier. In the simplest cases the axis of the embryo is 

 thus defined at once as lying between that point and the base of the 

 suspensor. The whole embryo is thus primarily a spindle-like body, and 

 this may be held to have been the primitive condition for them all. 



But this simple form is subject to early modifications, which disguise 

 the position of the axis by delaying its apical growth, and by distorting 

 the form : so much so that the position and identity of the apex is liable 

 to be lost. The least distorted types are those of L. Selago (Figs. 183, 184) 

 and Phlegmaria (Fig. 185), and of S. spitiulosa (Fig. 190), all plants which 

 are relatively primitive in their genera as recognised by the characters 

 of the mature sporophyte. In L. Selago and Phlegmaria no haustorial 

 swellings exist. The early development of the single cotyledon at first 

 throws the apex of the axis to one side, but this is rectified later when 

 the second leaf appears on the side opposite to the first. The apex thus 



