VITTARIA LINEATA 201 . 
mesophyll of the lower side is more loosely arranged and contains 
less chlorophyll (f. 776). 
The epidermis is composed of elements with very thick walls 
except in the grooves (f. 714-119). A reduction in size is 
noticeable over the middle and lateral veins and at the margins of 
the leaves. Chlorophyl is to be found in the epidermal cells in 
the grooves but seldom elsewhere. The epidermis of the grooves 
is uniformly thin-walled and gives rise to stomata and glands ; the 
latter are also found over the entire surface of the sporophyll. A 
narrow band of two to six rows of cells immediately external to 
the bundle underlying the groove bears the sporangia and para- 
physes (f. 723e). One to four layers of prosenchymatous cells 
are to be seen underneath the epidermis of the upper side of 
the leaf (f. 7746). Similar elements are to be found on the 
lower side of the leaf between the grooves, and these cells 
are somewhat larger than those of the upper surface (f. 71160). 
These hypodermal prosenchymatous elements are generally free 
from chlorophyl and are separated by numerous large air-spaces, 
giving the region a light gray color. The absence of these ele- 
ments in some instances cannot be accounted for. 
SCALES. — Scales are most abundant on rhizomes and least so 
on the leaves produced earliest (f. 72-74). The first leaf formed 
may bear but one scale while the later leaves may be covered 
with them. The scales are never more than one cell in thick- 
ness and nine or ten in width, attaining a length of 5 mm. and a 
breadth of .4-1.1 mm. The growth of these structures is long- 
continued and the basal cells may be in a state of division while 
the apical portion may be withered and brown (f. 75-82). The 
thin walls of the younger cells at the base measure 1.9 a in 
thickness which later increases to as much as 19.2 u, which is a 
most noticeable multiplication. This thickening is confined to 
the cross walls, the outer walls not participating in this increase 
(f. 76a, 81, 82a). The thickened walls show great numbers of 
dots or minute papillae, which are most numerous in scales borne 
on the stems and are not easily found in scales of the earlier leaves. 
Both the thickening of the walls and the formation of papillae are 
less marked in the marginal region of the scales. 
STOMATA. —Stomata are present only in the grooves of the 
