522 BOTANY OF THE LIVING PLANT 



though the Angiospermic development is relatively recent, the Seed- 

 Habit was initiated in the very remote past. 



It has been pointed out that the Pteridophyta retain their primitive 

 zoidiosamic fertilisation, a fact that has tended to restrict their 

 spread on exposed land-surfaces. Also it has been noted that certain 

 of them have taken the forward steps to heterospory, and to the 

 retention of the megaspore upon the parent plant. Both of these 

 essential steps towards the Seed-Habit can be adopted while still 

 retaining zoidiogamic fertilisation. It will be seen later that some 

 Seed-Plants still carry out their fertilisation by means of freely moving 

 spermatozoids, thus giving strong evidence of their Descent from a 

 Pteridophytic source. But the vast majority of Seed-Plants have 

 adopted the siphonogamic mode of fertilisation, by means of a pollen 

 tube, as already described for the Angiosperms in Chapter XVII. 

 This finally emancipated them from the embarrassing tie which 

 zoidiogamy imposed. Combining the Seed-Habit with siphonogamic 

 fertilisation the Seed-Plants became in actual fact Plants of the Land, 

 independent of water except such as can be extracted from the soil 

 by their roots. This was probably the chief factor leading to their 

 present dominant position. 



The Equisetales. 



The Equisetales, or Horse-tails, can only be briefly described here, though 

 they should not be omitted : for they figured largely as the Calamarians in 

 the primary rocks from the Devonian Period onwards, while the Class survives 

 in the cosmopolitan genus Equisetum. Whereas the Calamarian fossils often 

 attained tree-like proportions, the living types are relatively small. Never- 

 theless the general organisation of the Class is very uniform, the stem being 

 dominant and the relatively small leaves disposed upon it in successive 

 whorls with intervening internodes. This type of construction is sometimes 

 described as " articulate." It is shared by another Class of fossils, the Spheno- 

 phyllales : but these are all extinct. 



As in the Ferns, the sporophyte of the Equisetales is the substantive plant, 

 while the gametophyte is relatively small. The habit of an Equisetum is seen 

 in Fig. 413 a, with its webbed leaf-sheaths present alike on the rhizomes and 

 on the aerial shoots : both of these may be branched. Buds and related roots 

 arise at the nodes. The result is a certain uniformity of the microphyllous 

 habit, its differences depending mainly upon the degree of development of 

 the branching. A solitary distal fertile cone or strobilus may be seen on 

 each fertile branch. It consists of a central axis bearing lateral sporangiophores, 

 disposed less regularly than the leaves. Each sporangiophore supports a 

 number of large sporangia pendent from its peltate end. 



Anatomically the internode presents in transverse section a well-marked 

 epidermis with stomata leading to a broad, well-ventilated and photosynthetic 



