8 4 
Oliver —On Physostoma elegans , Williamson , 
The frequency curve embodying these data expressed as percentages 
has been plotted in Text-fig. 4. 
A similar curve based on countings of the petals of 1350 flowers of 
Sempervivum Funkii 1 — which vary round eleven — is placed beside it for 
comparison. The Sempervivum - curve also represents percentages. 
Text-fig. 4. Frequency curves, representing numerical variation of the ribs in Physostoma (on 
the left) and of the petals in Seinpervivum Funkii (on the right). The numbers are expressed as 
percentages in both cases. 
The close agreement in type between these curves points to the 
essential identity of the phenomena of numerical fluctuation in palaeozoic 
times and at the present day. 
The various organs and tissue-systems of Physostoma will now be 
described in detail. 
III. Details of Structure. 
In setting out the structural details of Physostoma , two principal 
regions are distinguished — the central or sporangial body of the seed and 
the envelope or integument by which the former is surrounded. These two 
regions are separate at the apex only ; throughout the rest of the seed they 
are completely confluent. 
In a good many seeds of this type the fusion is so complete that no 
anatomical feature stands out which can serve to indicate the position of 
the true boundary. In cases of this kind it is convenient to have recourse 
1 The Sempervivum-countings have been taken direct from Klebs’s ‘ Ueb. kiinstliche Meta- 
morphosen’, Stuttgart, 1906, p. 13. 
