88 THE MORPHOLOGY OF PTERIDOPHYTES 



Species usually have a single tetrahedral apical cell.^* Leaf 

 primordia are formed very close to the stem apex and, in 

 some species, appear to arise from a single cell. They give 

 rise to typical microphylls, receiving a single vascular bundle 

 which continues into the lamina as an unbranched vein. The 

 ligule, which is present on every leaf and sporophyll, appears 

 early in their ontogeny and develops from a row of cells 

 arranged transversely across the adaxial surface near the base 

 of the primordium. When fully grown it may be fan-shaped 

 or lanceolate and has a swollen *glossopodium' sunken into 

 the tissue of the leaf. There is much variation, according to 

 species, in the structure of the lamina of the leaf, for some 

 species possess only spongy mesophyll, while others have a 

 clearly defined palisade layer also. In some, the cells of the 

 upper epidermis and, in others, some of the mesophyll cells 

 contain only a single large chloroplast, a feature which is 

 reminiscent of the liverwort Anthoceros. In other species, all 

 the cells of the leaf contain several chloroplasts. There is 

 much variation, also, in the occurrence of stomata, some 

 species being amphistomatic and others hypostomatic. 



Early stages in the development of the sporangia in 

 Selaginella are very similar to those in Lycopodium, and there 

 is a similar range of variation in the location of the primor- 

 dium. Thus, in some species, it arises on the axis, while in 

 others it arises on the adaxial surface of the leaf, between the 

 ligule and the axis. However, at maturity the sporangium 

 comes to lie in the axil of the sporophyll. The first division 

 is periclinal and gives rise to outer jacket initials and inner 

 archesporial cells. The jacket initials divide further to pro- 

 duce a two-layered sporangium wall and the archesporial 

 cells produce a mass of potentially sporogenous tissue, sur- 

 rounded by a tapetum. In microsporangia many cells of the 

 sporogenous tissue undergo meiosis to form tetrads of 

 microspores but, in the megasporangia of most species, all 

 the sporogenous tissue disintegrates, except for one spore 

 mother cell, from which four megaspores are formed. Some 



