CHAPTER XI. 



THEORY OF THE STROBILUS. 1 



THE term strobilus is commonly applied to those fertile spikes with small 

 appendages which are found in a terminal position on the shoots of many 

 Pteridophytes. The construction of the vegetative region below the strobilus 

 in these plants is on essentially the same plan as the strobili themselves, 

 but without the sporangia. The similarity of the two regions, as well as 

 the absence of any definite limit between them, is demonstrated with 

 peculiar clearness in the case of Lycopodium Selago, as shown in the 

 Frontispiece. The absence of the sporangia in the vegetative region may 

 be accounted for on the ground of abortion. If this be so, the structure 

 of the strobilus will be the prototype for the vegetative shoot, and any 

 theory of its origin with its appendages should cover that of the vegetative 

 shoot as well. 



The strobiloid condition was common among the earliest Pteridophytes 

 of which there is any fossil evidence : there is thus a probability that it 

 was a relatively primitive state. It is characteristic of the Equisetales, the 

 Lycopodiales, and the Sphenophyllales, which are all relatively micro- 

 phyllous ; but the same type of construction is also traceable in radially 

 constructed megaphyllous forms, and it will be shown later how the Ophio- 

 glossales, and even the large-leaved Filicales, may be referred back to the 

 strobiloid type of construction, but with the appendages developed to an 

 inordinate size. 



The strobilus, in any of the small-leaved Pteridophytes, consists of a 

 relatively bulky axis, endowed with more or less continued apical growth, 

 and terminated by an apical cone, upon which the appendages arise 

 laterally (compare Fig. 67, p. 125). Those appendages appear in regular 

 acropetal order, below the apex of the axis ; and they arise exogenously, 

 as more or less massive outgrowths of the tissue of the apical cone (Fig. 71). 

 They have these characters in common in all cases, and, according to the 



1 This and the following chapters are largely based upon an address ^iven at the 

 International Exposition at St. Louis, U.S.A., 1904. 



