LECTURE VITI. 



FORMS AND SYSTEMS OF TISSUE : EPIDERMAL TISSUE AND 

 VASCULAR BUNDLES. 



By the term ' cell-tissue ' is designated in general the cellular structure of 

 plants. In particular, however, we understand by a form of tissue, a layered, fibriform, 



or other mass of cells, which, in their 

 growth and other physiological rela- 

 tions, present a certain agreement, 

 and are distinguished from the other 

 neighbouring masses of tissue. It 

 commonly happens that several forms 

 of tissues are again connected with 

 one another ; so that they constitute a 

 whole of definite physiological cha- 

 racter. Such a union of tissues is 

 termed a ' system of tissues.' Before 

 entering more closely into the descrip- 

 tion of tissues and systems of tissues, 

 however, it is perhaps desirable to 

 learn how the expression ' tissue ' in 

 ■general has been introduced into 

 vegetable anatomy, and later also 

 into the histology of animals. In 

 this expression we have in fact to 

 do with a historical curiosity; since 

 it is clear from what has been already 

 stated, that the cellular structure of 

 plants possesses very little resem- 

 blance to what are usually called 

 tissues, such as linen, cloth, or 

 Brussels lace, &c. Nevertheless, the 

 expression cell-tissue depends upon an erroneously assumed resemblance of cellular 

 structure to the things mentioned. One of the first founders of vegetable anatomy. 



Fig. io8.— Transverse section of the shoot -axis of Selaginella ina- 

 qtiali/olia. The epidermis and several layers of external tissue pos- 

 sess dark-coloured thick cell-walls ; the fundamental tissue, with thinner 

 walls, envelopes three libro-vascular bundles, which are separated from 

 it by large intercellular spaces {^). 



