ELEMENTARY STRUCTURE. 



41 



The epidermis covers all parts of those plants upon which it is 

 found which are directly exposed to the air except the stigiaa, and 

 it is in all cases absent from those which Hve under water. In 

 the Fungi, Algre, and Lichens, it is altogether wanting. The 

 young branches of trees are always covered with epidermis, 

 which is replaced at a subsequent period by a layer or layers 

 of cork. The roots of plants are invested by a modified epi- 

 dermal tissue to which the term Epiblema has been given by 

 Schleiden. This consists of ceUs with thin walls, without stomata, 

 but possessing cellular prolongations externally, called Hairs, 

 which will be described hereafter (p. 121 ). The inner surface of 

 the ovary, the canal of the style, and the stigma of Flowering 

 Plants are also covered by a modified epidermis, resembling 

 epiblema in its general characters, to which the name of Epithe- 

 lium has been given by Schleiden. 



b. Cuticle. — This consists generally of a thin pellicle, which 

 covers the entire surface of the ^r- g^ 



epidermal cells (Jigs. 82 a. and 

 84), with the exception of the 

 openings called stomata. It 

 forms a sheath also over the 

 hairs. The cuticle has no 

 cellular structure, but is a per- 

 fectly homogeneous membrane. 

 It is frequently prolonged into 

 the openings of the stomata, 

 and from thence into the passages which conmionly exist between 

 the sides of the cells below the epidermis (Jig. 86), and may then 

 be separated by boihng in nitric acid as a somewhat funnel- 



Fig. 84. Cuticle of the Cabbage, showing 

 that it is perforated by the stomata, and 

 forms sheaths over the hairs. 



Fig. 85. 



Fig. 86. 



Fig. 85. Cistome from Cereus peruvianttg. After Gasparrini F'ig. 86. Cis- 



tomes of Ornithogalum nutans, ramifying in the intercellular passages beneath 

 the epidermis, and becoming connected with one another. After Gasparrini. 



shaped bag (fig. 85). To this prolongation of the cuticle, which 

 Gasparrini first described, and which he wrongly regarded as a 

 peculiar organ, he gave the name of Cistome. 

 In rare cases, the cuticle, which is generally a very thin pelli- 



