CHAP. I. 



VASCULAR TISSUE. 



21 



becoming visible; and, finally, they may sometimes be de- 

 tached from the tissue (Plate II. fig. 7.), or fall away spon- 

 taneously. In the latter case they leave a hole in the tissue at 

 the place where they grew; and holes thus occasioned misled 

 Kieser into the belief that the woody fibre of Ephedra was 

 really pierced with pores of considerable magnitude. An 

 illustration of the manner in which these perforations are 

 caused will be found in Plate II. fig. 7. 



A different explanation is given by Mohl, whose observa- 

 tions have been confirmed by Unger. In the o^^nion of these 

 anatomists, the supposed glands of coniferous tissue are circu- 

 lar spaces, where the membrane of the tube becomes abruptly 

 extremely thin; and it is said that transverse slices of coni- 

 ferous wood, made at an angle of 45^, demonstrate the fact. 



Adolphe Brongniart has rightly stated, that there exists in 

 Gnetum Gnemon a form of tissue exactly the same as in 

 Coniferae. ( Voyage de Freycinet) In a species of that genus 

 collected in Tavoy by Dr. Wallich, the glands are very con- 

 spicuous. (Plate II. fig. 5.) 



Woody tissue constitutes a considerable proportion of the 

 ligneous'part of all plants; it is common in bark, and it forms 

 the principal portion of the veins of leaves, to which it gives 

 stiffness and tenacity. 



Sect. III. Of Vascular Tissue, 

 6 7 8 9 



10 \ 



Vascular tissue consists of simple membranous tubes ta- 

 pering to each end, but often ending abruptly, either having 



c 3 



