6i8 A TEXTBOOK OF THEORETICAL BOTANY 



SPHENOPSIDA 



This series of the Pteridophyta is marked by the very sUght development 

 or absence of branching in the main axes and by the clearly jointed stems on 

 which microphyllous leaves and lateral branches are borne in whorls. The 

 members are all homosporous with stalked sporangia which are aggregated 

 into strobili. The gametophytes of the living types are sub-aerial and green. 

 The jointed character of the stems has led to their being named the Articulatae 

 by many authors. 



The Sphenopsida include the following two orders : — 



1. Equisetales. 



Equisetaceae : Living plants belonging to the single genus 



Eqiiisetum. 

 Calamitaceae : Carboniferous fossils. 



2. Sphenophyllales. 



Sphenophyllaceae : Carboniferous fossils. 



EQUISETALES 



The Equisetales are Pteridophyta which are characterized by hollow, 

 jointed stems, with very small, simple leaves, arranged in whorls. The 

 sporangia are borne in groups attached to a common stalk or sporangiophore, 

 and these are arranged in whorls to form terminal cones or strobili. 



The Calamitaceae were large forest trees which formed a dominant part 

 of the vegetation in Palaeozoic times but became extinct at the close of that 

 era. In structure they resemble very closely the Equisetaceae of the present 

 day, and there can be little doubt that the living types oi Equisetiim are the 

 direct decendants of this family. 



There is a marked parallelism between the fossil Lepidodendrons and 

 the present-day Lycopodiums on the one hand, and the Calamites and the 

 living Equisetums on the other, for both groups have decreased in size in 

 the course of evolution and their structures have become simplified, as 

 for example in the absence of secondary thickening in the modern forms, 

 which was well developed in the Carboniferous ancestral types. At that 

 period the two fossil groups formed a major part of the vegetation, in so 

 far as we know it, while the modern representatives are relatively 

 inconspicuous plants. 



We shall describe here one example of the order, Eqiiisetiim arvense, and 

 return to a consideration of the fossil representatives in Volume IIL 



Equisetum arvense (The Field Horsetail) 



There are several species of Horsetail which occur commonly in this 

 country. Equisetum arvense (Fig. 629) is found in waste and gravelly places. 

 In general habit the species show a strong resemblance to one another, all 



