3rp 



VASCULAR CRYPTOGAMS. 



down like an apron from the apex of the stalk (Fig. 261, B, C, B). Its growth 

 now proceeds underground, so that it does not begin to unfold till the third spring, 

 M^hen it is raised above ground by the elongation of the leaf-stalk. The whole 

 of the leaves of a rosette of Aspidiiim 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 Mertensia, 

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



Fig. 261. — Pteris aqiiilijia; 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, 

 7u 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 witli the transverse section of the stem st, bs and / as in ^; D the lamina of a leaf in the 

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

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

 g fibro-vascular bundles. {A, B, C natural size.) 



pinnae (and when the pinnation is compound 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 only incompletely developed in a succeeding 

 period of vegetation, and then again in the same manner. This intermittent 

 development of the leaves may apparently extend over many years (see Braun, 

 'Rejuvenescence,' p. 114). According to Mettenius, the lamina of some Hymeno- 

 phyllaceae is capable of unlimited development, and is annually renewed. In Ly- 

 godium the primary branches of the lamina remain also in a bud-like condition 

 at the end after the formation of each pair of pinnae of the second order, while the 

 rachis of the leaf grows without limit and resembles a twining stem. 



The branching of the lamina of Fern-leaves is not unfrequently forked in the 

 mature state, as in Platycerium, Schizaea, &c. ; but Hofmeister refers also the pinnate 

 forms to dichotomous branching at the commencement, which becomes sympodial 

 with further development, a right and left fork being alternately weaker in its growth. 



