38 ORGANOGRAPHY. 



which have originally no pi'oper membrane, but acquire one 

 subsequently by the deposition of matter of varying thickness 

 from the secretions which they contain. More recently again, 

 Schacht has advanced an opinion that they are all liber-cells. 

 Further investigation is necessaiy before we can therefore 

 pronounce positively as to their origin or nature. Our own 

 opinion is, that they vary in their nature in different plants, and 

 in different parts of the same plant. Trecul has lately endea- 

 voured to prove that the laticiferous vessels are in direct con- 

 nexion Avitli the spiral, but we cannot satisfy ourselves of the 

 truth of this. 



We have now described all the different forms of cells, and the 

 modifications they undergo so as to form vessels. The different 

 kinds of vessels and woody tissue or fibres, are more or less 

 united together, and have always a tendency to develope and 

 arrange themselves in longitudinal or vertical l)undles in the parts 

 of the plant where they are found, and thus they may be readily 

 distinguished from the ordinary parenchyma in which they 

 are placed, both in their forms and mode of elongation. We 

 thus find it very convenient to speak of the bundles formed by 

 the combination of the fibres and vessels under the collective 

 name of Fibro-vascular Tissues, or the Fihro-vascular, or Ver- 

 tical, or Longitudinal System, to distingaiish them from the ordi- 

 nary cellular tissue, wliioh constitutes the Farcnchymalous, or 

 common Cellular, or Horizojital System. 



4. Epidermal Tissue. — In the higher FloAverless, and gene- 

 rally in tlie Flowering Plants, the cells situated on tlie surface of 

 the different organs vary in shape and in the nature of their con- 

 tents from those placed beneath them, and form a firm layer 

 which may commonly be readily separated as a distinct mem- 

 brane or skin. To this layer the term Epidermal Tissue is given. 

 This is generally described as consisting of two parts: namely, 

 of an inner portion caUed the Epidermis, and of an outer thin 

 pellicle to which the name Cuticle is given. By Carpenter, 

 howev(?r, and some other authors, tliese terms arc used in pre- 

 cisely the reverse sense ; thus Cuticle to indicate the Epidermis, 

 and vice versa. We use the term as first mentioned, because 

 more in accordance Avith general usage, although it must be 

 admitted ihut the name I<^])idcrmis, signifying as it does, amem- 

 l)rane lying upon the dermis or skin, would be more appropriate 

 if applied in tlie latter sense. 



a. Epidermis. — This consists of one or more layers of cells, 

 firmly united together ])y their sides, and forming a continuous 

 membrane, except at the i)oints where it is perforated by the 

 Stomnta, ])r('sently to be described {fuj. 91). These cells are 

 generally of u ilattened tabiiliir character, the sides of which 

 vary much in their outhne ; thus in the epidermis of the /n.«, 

 and many other Monocotyledons they are elongated hexagons 



