LYCOPSIDA 91 



a vertical position. S. Kraussiana (Figs. 14U and 14V) likewise 

 has a diaphragm, but in this species the venter of the arche- 

 gonium gradually extends through it (a), so carrying the 

 embryo with it into the centre of the prothallus. The outerhalf 

 of the dividing zygote provides, not only the vestigial suspensor, 

 but also the foot. The archegonium of S. Galeottii behaves in a 

 similar way, but the embryo is different (Fig. 14W) in that 

 the outer half provides the suspensor, the foot and also the 

 root. S. denticulata (Fig. 14X) has the various parts of the 

 embryo disposed as in S. Martensii (i.e. the root lies between 

 the suspensor and the foot) but they are derived in a com- 

 pletely different way, for they all come from the outermost 

 half of the dividing zygote. 



Such extraordinary variations as these are very puzzHng 

 and have occupied the thoughts of many morphologists. 

 Some have held that the presence of a well developed sus- 

 pensor is a primitive character and that the reduction of this 

 organ in some species is a sign of relative advancement. 

 Its reduction seems to be correlated with the transference 

 of its function to the venter of the archegonium, and 

 this would certainly seem to be an advanced condition. As 

 to the *foot', all that can be said is that it has Httle reality 

 as a separate organ, since it can apparently be formed from 

 various regions of the embryo and may even be dispensed 

 with altogether. 



Selaginella is pecuUar among pteridophytes for its low 

 chromosome numbers, n = 9 being the commonest number, 

 and its chrom.osomes are minute. 



Selaginellites is the genus to which are assigned all fossil 

 remains of herbaceous lycopods that are known to have been 

 heterosporous. The recent examination of Selaginellites crassi- 

 cinctus'^^ is of particular interest, for within its cones were 

 found the megaspores Triletes triangularis, which have long 

 been known as one of the commonest spores in coal measure 

 deposits, but whose origin was hitherto unknown. This dis- 

 covery suggests that Selaginellites v^sls probably an important 



