THE LYCOPSIDA AND PTEROPSIDA 251 



the usual exhibition of a high degree of multiplication of the 

 sporangia which are often disposed on common stalks known as 

 sporangiophores. The branches instead of being truly axillary are 

 borne alternately with the leaves at the nodes. The Equisetales 

 may conveniently be divided into three families the Sphenophyl- 

 laceae, the Calamitaceae, and the Equisetaceae which are in all 

 probability related to one another in the order indicated by their 

 enumeration. The Sphenophyllaceae were forms in which the 

 central cylinder of the stem was protostelic. The cones consisted 

 of sporophylls presenting various degrees of complication. In the 

 simplest forms the sporangia were inserted singly on the stalks or 

 sporangiophores and were numerous for each sporophyll. In more 

 advanced types the sporangia became two or more for each spo- 

 rangiophore. The Calamitaceae, like the Sphenophyllaceae, are 

 organisms entirely extinct. They usually possessed the arboreal 

 habit and were invariably characterized by a siphonostelic central 

 cylinder. The sporangiophores bore four sporangia and were 

 variously related to the sporophylls. In the Equisetaceae are 

 included a number of genera, living and extinct, of herbaceous 

 habit and possessing so far as is known a simplified type of cone in 

 which sporophylls are represented by the sporangiophores alone. 



In the Pteropsida, marked by the general features enumerated 

 in earlier paragraphs of the present chapter, there are three large 

 subdivisions the Filicales, the Gymnospermae, and the Angio- 

 spermae. The first of these, as the name indicates, include the 

 fernlike forms that is, those in which the reproduction takes place 

 through unicellular bodies known as spores. In the Gymnospermae 

 true seeds are present which in every case are equipped to receive 

 the microspores; and these after germination effect fertilization 

 either by means of antherozoids (Archigymnospermae) or through 

 the agency of pollen tubes (Metagymnospermae). In the last and 

 (in the present epoch) most important subdivision of the Pterop- 

 sida, the Angiospermae, the pollen is received on the apex of the 

 closed megasporophyll and no longer falls upon the seed. Fertiliza- 

 tion is invariably by means of a pollen tube. The habit of the 

 angiosperms is either arboreal or herbaceous, and the fibrovascular 

 tissues show a high degree of specialization. 



