86 



BOTANY 



PART I 



than a metre, and in plants the woody portion of which conducts water only by 

 vessels, the vessels have an average length of only ten centimetres. The length of 



Fig. 93. — Part of the wall of sieve- 

 tubes of Pinus sylvestris, in tan- 

 gential section, after treatment 

 with chloroiodide of zinc ; A, be- 

 fore, B, after formation of the 

 callus-plate ; C, portion of a sieve- 

 tube no longer in activity. ( x 540. ) 



Fig. 94. — Lower third of a scalari- 

 form vessel from the rhizome of 

 the common Bracken Fern, 

 Pteris aquilina. t, Transversely 

 elongated pits in the lateral 

 walls ; q, scalariform perfora- 

 tions of the terminal wall. (After 

 De Baey, x 95.) 



an individual vessel is denned hy the presence of transverse walls, which are not 

 perforated except by bordered pits. 



Tissues 



A continuous aggregation of cells in intimate union is called a 

 tissue. The origin of vegetable tissues is, in general, attributable to 

 cell division. It is only in the Fungi and Siphoneae that a tissue 

 arises through the interweaving of tubular cells or cell filaments (Figs. 

 95, 96). In such cases, where the filaments are so closely inter- 

 woven as to form a compact mass of cells, the apparent tissue thus 

 formed has the same appearance as the tissues of higher plants 

 (Figs. 97, 98). 



The mutual interdependence of the cells of a tissue is manifested 

 both by the conjunction of their pits (Figs. 81, 83, 84), and by the 

 general similarity of their wall thickenings. 



The PROTOPLASTS OF MOST CELLS ARE DIRECTLY CONNECTED "WITH 

 ONE ANOTHER BY MEANS OF EXTREMELY DELICATE THREADS OF 



