XXXI LEAVES AND ROOTS 421 



the parts of a child's puzzle. Among them are found at 

 intervals pairs of sausage-shaped cells {gd. c) placed with 

 their concavities towards one another so as to bound a 

 narrow slit-like aperture {st). These apertures, which are 

 the only intercellular spaces in the epidermis, are called 

 stomates : the cells bounding them are the guard-cells, and 

 are distinguished from the remaining epidermic cells by the 

 possession of a few chromatophores. 



The mesophyll, which as we have seen occupies the whole 

 space between the upper and lower epidermis, is formed of 

 thin-walled cells loaded with chromatophores (k, ms.ph) and 

 therefore of a deep green colour. The cells in contact with 

 the upper epidermis are cylindrical, and are arranged verti- 

 cally in a single row : those towards the lower surface are 

 very irregular both in form and arrangement. I^rge inter- 

 cellular spaces (/. c. sp) occur between the mesophyll-cells 

 and communicate with the outer air through the stomates. 



The leaves arise as outgrowths of the distal or growing 

 end of the stem, each originating from a single segmental 

 cell of the apical cone. 



The fern is the first plant we have yet considered which 

 possesses true roots, the structures so-called differing funda- 

 mentally from the simple rhizoids of Nitella and the mosses. 

 Instead of being mere linear aggregates of cells, they agree 

 in general structure with the stem from which they spring, 

 consisting of an outer layer of epidermis within which is 

 parenchyma strengthened by bands of sclerenchyma and by 

 a single vascular bundle in the middle. The epidermic cells 

 give rise to unicellular prominences, the root-hairs. 



The apex of the root, like that of the stem, is formed of 

 a mass of meristem in which a single wedge-shaped apical 

 cell (Fig. 110, M, ap. c) can be distinguished But instead 



