154 MANUAL OF BOTANY 



vascular strand passing up the centre. It contains no assimi- 

 lating tissue. 



The vascular cylinder of the root is monostelic ; the stele 

 contains three wood and three bast bundles and is invested by a 

 double -layered endodermis, from the innermost layer of which 

 the lateral branches spring. There is no pericycle. 



The sporangia are ovoid bodies, arranged with their long axes 

 pointing towards the stem. Each is derived from several 

 epidermal cells, the Equisetinae being eusporangiate. The 

 wall of the sporangium is one layer of cells deep, and these have 

 peculiar thickenings similar to those of the anthers of the higher 

 flowering plants. 



The spores are all alike, the Equisetinse being isosporous. 

 A perinium, or epispore, is developed, resembling somewhat that 

 of the spores of Marsilea. When the spores are mature, this 

 coating splits up into four bands which coil round the spore, 

 being attached to one point of its surface. These are known as 

 elaters, and they aid in the dissemination of the spores by virtue 

 of their hygroscopic qualities. 



When the spore germinates it produces a gametophyte or 

 prothallium very much like that of the Ferns, but somewhat 

 more irregular in contour. 



The prothallia are generally dioecious, producing antheridia 

 or archegonia, but not both. Both antheridia and archegonia 

 closely resemble in structure and development the corresponding 

 organs in the Fern. 



The embryo produced from the zygote has two cotyledons, 

 contrasting with that of the Ferns, which has only one. The 

 octants, with this exception, behave much as in the latter group. 



The Equisetinse were represented in Carboniferous times by 

 a very prominent group of plants now known under the name of 

 Calaniites. Some of these were heterosporous. Though much 

 like the existing forms, they showed a certain variety in the 

 mode of arrangement of the sporophj^ls. They differed very 

 greatly in anatomical structure on account of the bundles being 

 ojjen instead of closed. The band of cambium gave rise to 

 considerable amounts of secondary wood and bast, so that the 

 Calamites increased in thickness as do now the Dicotyledonous 

 flowering plants. 



