136 THE MORPHOLOGY OF PTERIDOPHYTES 



there is a pair of thin stipules which enclose the apical bud; 

 and the next leaf, when it begins to grow, has to break its 

 way through the thin sheath covering it. UnUke all other 

 living ferns their leaves are not circinately coiled when young. 

 In all three genera, the fertile fronds have two distinct 

 parts, the fertile part being in the form of a spike which 

 arises at the junction of the petiole with the sterile lamina, on 

 its adaxial side. The fertile spike is pinnately compound in 

 those genera with a compound lamina and simple in 

 Ophioglossum, where the lamina is simple. Its morpho- 

 logical nature has been the subject of some considerable 

 discussion in the past but is now generally thought to repre- 

 sent two basal pinnae which have become ontogenetically 

 fused, face to face (i.e. it is believed that some early ancestor 

 of the group had two fertile basal pinnae, whose primordia 

 became fused during subsequent evolution). Today, the only 

 evidence for the double nature of the spike lies in its vascular 



supply. 



The roots are peculiar in being completely without root 

 hairs, a feature which is possibly connected with their 

 mycorrhizal habit. 



Growth of the stem apex is from a single apical cell, and 

 its products are characteristically soft and fleshy, for they 

 are without sclerenchyma. The stem of the young sporehng 

 is protostelic, but soon becomes medullated. Later on, the 

 stem of Botrychium becomes solenoxyUc, i.e. there are leaf 

 gaps in the xylem, but not in the single external endodermis. 

 Ultimately, the appearance of a sporadic internal endo- 

 dermis may give rise to a rudimentary solenostele. Botrychium 

 is the only genus of living ferns to show secondary cambial 

 activity, and in some species it may give rise to a consider- 

 able thickness of secondary wood, composed of tracheids 

 and wood-rays. Rhizomes of Helminthostachys pass through 

 much the same stages of stelar organization, but the largest 

 specimens go one stage further and achieve true soleno- 

 stely, with an internal as well as an external endodermis. 



