32 DOUGLAS HOUGHTON CAMPBELL ON THE 



expansion of parts already formed, this process taking place with extreme rapidity in 

 strong contrast to the extraordinarily slow development of the earlier stages of the leaf. 



The apical cell of the leaf (PI. 6, fig. 7) is wedge-shaped, diftering, therefore, in ap- 

 pearance as seen from dift'erent directions. The segments are formed hy walls par- 

 allel to the lateral faces. Each segment is first divided into two cells by a transverse 

 wall; the main fibro-vascular bnndles of the leaf being developed from the inner of the 

 two cells, the mesophyll, and the epidermis from the outer. The outer cell is next further 

 divided by a wall parallel to the primary wall, and these two cells by walls parallel to 

 their outer surfaces, these outer cells constituting the marginal epidermis. In the older 

 leaves these marginal cells, although increasing rapidly in breadth, undergo no fuilher 

 division, the rest of the epidermis arising from the cells lying within these by walls par- 

 allel to the surface of the leaf. In the very young leaf, before there is any dififerentiation 

 of stalk and blade, the growing point is thicker and the marginal cells early undei-go 

 division by walls parallel to the surface of the leaf. 



Longitudinal walls are formed in the inner cells so as to transform them into a cylin- 

 drical mass of cells, those in the center being narrower and forming the procambium, the 

 others the young ground tissue and epidermis. In the blade, all of the structures devel- 

 oped from the central cells remain as the rachis, but in the petiole it is impossible to dis- 

 criminate between the ground tissue and epidermis developed from these cells and those 

 fi-om the outer ones of the segments. The cells of the procambium (PI. 6, fig. 7) 

 soon become easily recognizable by their greater length and more transparent contents. 

 In the lower part of the leaf two parts can be distinguished, separated bj' a mass of 

 shorter cells and these can be traced to the stem-bundle with which they are united, one 

 coming from each side of the foliar gap, or opening in the stem-bundle opposite the in- 

 sertion of the leaf. 



The differentiation into petiole and blade is caused, by the lateral growth becoming 

 more strongly marked in the latter, the cells dividing ra])idly by walls perpendicular to 

 the siirface of the frond, this being very strongly marked in the older leaf. 



As growth continues, the young ground-tissue of the petiole begins to show a divis- 

 ion into tAvo portions: the true fundamental tissue and the hy^Joderm. The cells form- 

 ing the latter are much narrower than those of the former and contain less chlorophyll, 

 so that they are easily distinguished from the parenchyma l3'ing beneath them at an early 

 stage. They have oblique end Avails, and increase ra})idly in length, the Avails also in- 

 creasing in thickness and finally becoming dark broAvn. When the hypodermal cells are 

 completed they form a sort of fibrous tissue. Outside of the hypodermal cells there is a 

 single layer of epidermal cells, diftering from them in their greater transverse diameter, 

 but Joeing shorter, resembling in shape more nearly the inner ground tissue cells. On 

 the older basal portion of the stipe, the epidermis, together Avith its aj)pendages, hairs 

 and scales, becomes dry and brown, forming with the hypoderma a sort of cortex pro- 

 tecting the inner soft parenchyma. No stomata are developed from the epidermis of the 

 petiole or rachis. 



By the time that the ground tissue becomes recognizable as such, the formation of A'cr- 

 tical Avails in its cells ceases, the subsequent increase in the numlier of cells being caused 

 by transverse Avails only, resulting in the formation of regular vertical i-ows of cells. In 



