FILICINEJE. 



431 



larkable, finding its parallel only among the Ophioglossaceae. In old plants of 



^kris aquilina the formation of the leaf commences fully two years before its un- 



)lding: — at the commencement of the second year only the leaf-stalk is as yet 



existence, about one inch high. Up to this period its growth has been effected 



Jy a single apical cell which is divided by oblique walls in alternating directions; 



is now carried on by a number of marginal cells which divide. The pinnae are 



'derived from the segments of the single apical cell; the veins are formed by the 



repeated divisions of the marginal cells (Sadebeck). In the summer of the second 



year the lamina arises for the first time at the apex of this rod-like body, and may be 



found hidden in the form of a minute disc beneath the long hairs. It immediately 



bends downwards at its apex, and hangs down like an apron from the apex of 



the stalk (Fig. 301, B, C, D). Its growth now proceeds underground, so that it 



does not begin to unfold till the third spring, when it is raised above ground 



by the elongation of the leaf-stalk. The whole of the leaves of a rosette of Aspidium 



Fig. 301. — Pteris aquilina; A the end of a stem st, the apex lying at ss ; by its side at b is the rudiment of a leaf, 

 bs the stalk of a leaf in the second year, at h its lamina enveloped by hairs, K a bud at the back of the leaf-stalk, 

 w roots ; B a young leaf in the second year, bs its stalk, / its small lamina with the hairs removed ; C longitudinal section 

 of a similar leaf, connected with the transverse section of the stem st, bs and /, as m. B ; D the lamina of a leaf in the 

 second year seen in front, i. e. on the upper side (X about 5) ; the first segments have begun to be formed ; E horizontal 

 longitudinal section of a branching of the stem, ss s's the two apices, a a brown epidermal tissue, b b brown sclerenchyma, 

 ^ fibro-vascular bundles. (A, B, C natural si2e.) 



Filix-mas have been in course of formation two years before their unfolding; 

 the leaf-stalk is in this case also formed in the first year, and the first formation 

 of the lamina takes place on the oldest leaves of the young rosette. 



The basifugal apical growth of the lamina of Fern-leaves is however most 

 conspicuous when it continually advances for a considerable time without attaining 

 a definite conclusion while the lower parts of the lamina have long been fully 

 developed, as in Nephrolepis. The periodical interruption of the apical growth 

 of the lamina already mentioned occurs in many species of Gleichenia and Merknsia, 

 where the development of the leaves remains stationary above the first pair of 

 pinnae, and when the pinnation is compound this is often repeated in the several 

 orders of branching, so that the apex, forming apparently a bud in the fork, either 

 remains altogether undeveloped, or is developed in a succeeding period of vegetation, 

 and then only incompletely. This intermittent development of the leaves may 



