278 Mr. A. Heufrey on the Progress of Physiological Botany. 



alone ; and although he is in accordance with most authors in 

 stating that the fronds of Alga? grow by their apices and 

 borders, there are also exceptions here — for instance in Lami- 

 naria digitata, where the new frond is produced by the expansion 

 and development of the stalk-like part of the old one. Passing 

 by these, it is evident that the leaves of Mosses and Hepaticce 

 differ widely from those of the Phanerogamia in general in their 

 development, though they bear considerable resemblance to such 

 as those of Utricularia, &c, and many of the Monocotyledons. 



The leaf of a Dicotyledon originates as a little papilla of cel- 

 lular tissue : if it is a lobed leaf, these lobes appear in succession ; 

 thus at first we see a little cone, then a three-lobed flattened pa- 

 pilla, next a five-lobcd, and so on ; and here it is difficult to say 

 how we shall prove, how in the five-lobed form the intermediate 

 lobes originated — whether they are new ones, or the two original 

 lateral lobes pushed up by two succeeding lobes — since we can 

 only make observations on separate leaves, not see them grow ; 

 but as it is clear that the papilla does grow at the base, becoming 

 narrowed into a petiole and pushing the whole of the blade up, 

 we have a right to assume that the leaf does in the first instance 

 develope at its base. But then we must not generalize for the 

 whole growth from this, since as soon as the petiole is distinctly 

 formed, the petiole and the lamina have distinct growth ; and now 

 the leaf in its expansion by the multiplication of its cells must 

 grow chiefly at its borders, since the centre of the base, that is, 

 the point of junction with the petiole, must retain its relative 

 position, and may therefore be considered as the point of de- 

 parture of all growth in the lamina ; so that as the apex and the 

 borders are subsequently at a greater distance than at first, they 

 must develope away from it in all directions, whether by mere 

 marginal and apical alone or by central development also, since 

 in the latter case the border must grow to make room for the 

 growth in the centre. Nageli says that the growth by expansion 

 of the individual cells commences at the apex, but it would very 

 often be difficult to distinguish whether this expansion at the 

 apex depends on development of cells or actual expansion of those 

 already formed ; he probably reasoned from analogy here in re- 

 gard to the Phanerogamia. Most experiments have shown the 

 expansion to be tolerably simultaneous throughout. 



The leaves of Monocotyledons, such as those of the common 

 bulbous plants at least, appear to develope chiefly, if not solely, at 

 the base. In those which have petioles there must be a differ- 

 ence, but in such we observe the growth or actual development 

 to continue longer in the petiole than in the blade. 



The forms of leaves differing so much even in the same spe- 

 cies, often in consequence of difference in the amount of paren- 



