THE HIGHER CRYPTOGAMIA. 397 



leaves the naked terminal bud above them becomes forked 

 (PL LVII, fig. 22). Now, or soon afterwards, the cells of 

 the lower portion of the axis of the second order undergo a 

 sudden and considerable longitudinal expansion, and thus 

 break through the small-celled upper half of the prothallium. 

 The two leaves spread out expand in length and breadth, 

 and become green. 



At the same time the two axes of the third order, into 

 which the end of the axis of the second order has divided, 

 commence their further development. Their cells multiply 

 rapidly in a longitudinal direction, and produce leaves. 

 The four longitudinal rows of leaves do not appear con- 

 temporaneously upon the shoots of the third order. A 

 lower first leaf appears without an opposite upper leaf. The 

 next leaf which is a lower leaf of another longitudinal row, 

 is also often without an opposite upper leaf. From thence 

 upwards the arrangement of the leaves is regularly 

 2^. The longitudinal expansion of the two shoots of 

 the third order (like that of the subsequent vegetative 

 shoots) commences, in Belqginella hortensis, for the first 

 time, when their ends begin to fork for the formation of 

 the axis of the fourth order, and the result is that these 

 leaves of the shoots of the third order are for some time so 

 closely crowded, as to appear upon a cursory examination 

 to belong to the axis of the second order, with whose two 

 leaves they seem to form angles of different divergence. 



The first adventitious root springs out of that side of the 

 axis of the first order which lies opposite to the place of 

 origin of the shoot of the second order. It corresponds in 

 development and structure most completely with those after- 

 wards produced. In Selaginella hortensis it is usually 

 first developed at a very late period, at the time of the 

 commencement of the longitudinal development of the 

 axes of the third order. Exceptions to this are of 

 rare occurrence. It appears much earlier on the germ- 

 plant of S. Marten si ; even whilst the embryo remains 

 within the lower layer of the prothallium (PI. LVII, 

 fig. 20). In the latter species the end of the primary axis 

 is far more fully developed than in the former. 



