148 DERMAL SYSTEM 



drop of water is placed upon a thallus, which has been allowed to dry 

 up until it is quite brittle, the moistened spot at onces becomes soft 

 and pliant. It should be stated that there are no openings in the 

 epidermis through which water could enter. The epidermis of the 

 lower side of the thallus, which is plentifully provided with hairs, has 

 an entirely or almost entirely unthickened outer wall ; the latter is 

 insoluble in concentrated sulphuric acid, wdiereas the lateral and inner 

 walls dissolve in this reagent. 



VI. ONTOGENY OF THE DERMAL SYSTEM. 



In considering the relations of the various dermal tissues to the 

 three primary apical meristems, namely, the protoderm, procambium and 

 fundamental meristem, discussion must be confined to the epidermis, 

 since cork as has been shown above always arises from the phellogen, 

 which is a secondary meristem. In the case of aerial organs the 

 epidermis almost always develops from the protoderm, which was 

 indeed for this reason termed the " dermatogen " by Hanstein. In 

 certain cases, however, a layer which is fully qualified, 

 both anatomically and physiologically, to rank as an 

 epidermis is ontogenetically derived from the funda- 

 mental meristem. s7 This abnormal relation is excellently 

 FlG ." 46 . illustrated by the epidermal cells surrounding the per- 



Epidermai ceii forations and indentations which regularly occur in the 

 SnL'oftTSr" fully-grown leaves of many Aroids. F. Schwarz has 

 forations ^in^ the described j^e or jgi n f the perforations in the case of 



Monstera dcliciosa as follows. When the young leaves 

 have reached a length of about 8 mm., the meristematic tissue dies and 

 turns brown within certain sharply circumscribed, but not otherwise 

 differentiated, areas situated between the secondary veins. The cells 

 which immediately abut upon these patches of dead tissue repeatedly 

 undergo division in a direction which is tangential with reference to 

 the edge of the patch, and give rise to a tissue resembling periderm. 

 The outermost cells of this marginal tissue then develop into a secondary 

 epidermis, which thus forms a narrow strip interpolated between the 

 upper and lower primary epidermal layers. Schwarz gives no details 

 as to the histological features of this secondary epidermis derived from 

 the fundamental meristem of the leaf. The author has therefore care- 

 fully compared the epidermis that bounds the edges of the marginal 

 indentations with the primary epidermis of the leaf-margin, especially 

 with regard to the detailed structure of the outer walls. He finds 

 that the agreement between primary and secondary epidermal cells is 

 complete in every particular. The outer walls of the secondary cells 



