

EMBRYOLOGY 361 



" righted " is then carried up together with the two leaves by the elong- 

 ating hypocotyl, while the first root appears laterally at its base. The 

 whole arrangement is relatively simple, but illustrates a slight degree of 

 distortion of the apex, which is, however, temporary only. 



In the c/avatum-type (Figs. 170, 186) the hypobasal tier is enlarged, 

 and curved to one side, with correlative late differentiation of the epibasal 

 region, and absence at first of localised intercalary growth. The orien- 

 tation of the foot is not constant, but it is directed towards the chief 

 source of nutriment, a fact which indicates its opportunist character. In 

 the epibasal region the apex of the axis is clearly of central origin, 

 between the two small but equal cotyledons, which appear relatively late. 

 The root originates in a position corresponding to that of the former 

 type. The whole embryo may be held to be a biologically intelligible 

 modification of the Se/ago-type consequent on the underground habit of 

 the large mycorrhizal prothallus. The enlargement of the haustorial foot 

 leads correlatively to slow development of the epibasal region, while the 

 first leaves, having no nutritive function, are not hurried on in their 

 development so as either to distort or to produce correlative reduction of 

 the apical cone. 



In the cernuum-type (Figs. 187, 188) the tuberous swelling is not in the 

 hypobasal but in the epibasal region, and it profoundly disturbs its develop- 

 ment. The biological cause of the swelling, which is extra-prothallial and 

 liable to repetition, may be the intrusion of the symbiotic fungus which is 

 present, or there may be some other reason for the tuberous development, 

 associated as it is with the late origin of the root. But whatever the 

 cause, the form is such as might be expected in a secondary tuberous 

 modification of a green leafy shoot of a young Lycopod. The bulky 

 development below, and the rapid enlargement of the assimilating leaves 

 act correlatively in keeping the stem-apex inconspicuous. Its identity 

 throughout the embryogeny of L. cernuum itself is not yet demonstrated ; 

 but in Phylloglossum the definitive apex of the shoot has been seen to 

 coincide with the apex of the tuber (Fig. 189) ; it is therefore probable 

 that in the embryogeny of L. cernuum the apex of the axis is present 

 in the very young embryo in the usual position, but has escaped recog- 

 nition owing to its correlative diminution. The protophylls would then 

 be leaves of the normal type, altered in relation to the gouty habit of the 

 axis which bears them, and disposed in an apparently irregular and isolated 

 fashion upon the swollen axis. The root is long deferred, perhaps in 

 relation to the mycorrhizal habit; but when it does appear, its relation to 

 the leafy shoot is like that which it has in the other types of the genus. 

 The normal leafy shoot and the root-system, thus delayed by the gouty 

 interlude called the " protocorm," ultimately continue their development 

 as in other Lycopods. Thus the different types of the genus appear to 

 start alike, and when established in the soil continue alike, but show 

 divergent intermediate phases. The simple development of Z. Selago and 



