68 



BOTANY 



tissue. The wood of the stem, and the framework, or veins of the 

 leaves, belong to the vascular system. The mechanical elements of 

 the vascular bundle are of two kinds, Fibres either wood or bast 

 fibres and tracheary tissue. The latter is also the principal water- 

 conducting tissue of these plants, and may be composed either of 

 Tracheids, which are single elongated cells, or Vessels, which are 

 rows of cells in which the transverse partitions have disappeared. 

 Both forms of tracheae, when mature, are destitute of living contents, 

 and their walls are marked by rings, spirals, reticulations, or pits, 

 due to unequal thickening in the growing wall. 



FIG. 39. A, cross-section of a vascular bundle from the scape of Iris Florentines 

 (X 250) ; t, tracheary tissue; ph, phloem. B, longitudinal section of the same; 

 t, spiral, t', reticulate vessels; s, a sieve-tube. 



In some of the lower plants, like some Seaweeds, firmness is given 

 to the plant by great thickening of the walls of the superficial cells, 

 such as occurs in many forms which are exposed to the heavy surf. 

 Others, like the calcareous Algae, attain the same end by a heavy 

 deposit of lime in their outer cells. 



Protective Tissues. All of the superficial cells of plants exposed 

 to the air are provided with a heavily cutinized membrane, which is 

 especially developed in plants of dry regions. This thick cuticle 

 prevents excessive loss of water from the delicate inner tissues. 

 The layers of cork-cells in the stems of woody plants serve the same 

 purpose. 



