PTERIDOPHYTA 247 



Embryo. The embryo of Isoetes differs from that of other living Lyco- 

 podiiiiae in lacking a suspensor. The fertilized egg undergoes a trans- 

 verse division, but both segments take part in the formation of the 

 embryo proper. A quadrant stage is organized. It seems probable that 

 the two outer cells form the foot, one of the two inner cells the root, and 

 the other inner cell the leaf. As the embryo develops, a greater growth 

 on one side causes it to curve until it finally becomes inverted, the foot 

 lying below and the root and leaf above (Fig. 204F). The stem makes its 

 appearance later between the root and leaf. It may originate from either. 



Summary. Isoetes has an unbranched tuberous stem bearing a rel- 

 atively few large leaves, each of these having a ligule. The vascular sys- 

 tem of the stem is a greatly reduced protostele, amphicribral in organ- 

 ization, and -^-ith secondary thickening. A definite strobilus is not 

 organized, unless the whole plant be considered as one. Isoetes is hetero 

 sporous, the megasporangia producing many megaspores, the micro- 

 sporangia a much greater number of microspores. Trabeculae are formed 

 in both kinds of sporangia. There is no regular dehiscence. The male 

 gametophyte, formed inside the microspore, consists of a single prothallial 

 cell and a single antheridium, the latter producing four large multiciliate 

 sperms. The female gametoph\i:e, developed inside the megaspore, has 

 considerable vegetative tissue and one to several archegonia. These have 

 only one neck canal cell. The embryo is \\'ithout a suspensor. Although 

 having a number of characters in common with the other Uving lycopods, 

 Isoetes occupies an isolated position because of its general habit, leaves, 

 multiciUate sperms, and absence of a suspensor. 



3. EQUISETINAEi 



The Equisetinae, like the Lycopodiinae, are a group of ancient origin 

 and were much more abundant and diversified during the Paleozoic than 

 they are today (Fig. 258). They are characterized by jointed, longi- 

 tudinally fluted stems bearing mostly small, simple leaves arising in 

 whorls and usually united to form a sheath around each node. This 

 cycUc arrangement of leaves is in marked contrast to the spiral arrange- 

 ment characteristic of other pteridophytes. The sporangia are mostly 

 numerous and borne on the underside of stalk-like sporangiophores that 

 are nearly always organized to form a compact strobilus. Of the four 

 orders — the Hyeniales, Sphenophyllales, Equisetales, and Calamitales — 

 only the third has Uving members. 



1. Hyeniales 



The Hyeniales are the oldest and most primitive order of Equisetinae, 

 in some respects resembling the Psilophytales. They comprise two 



* Also called Sphenopsida or Articulatae. 



