110 



BOTANY 



the venation. In many parallel-veined leaves the bundles are easily 

 isolated. This is often done accidentally, as when, for example, in 

 picking a leaf of Plantain (Planfayo media) a pull is given at the same 

 time. 



Special strands of tissue serving for the transport of substances 

 through the plant are found in the more highly differentiated Thallo- 



Fio. 121. Transverse section of a vascular bundle from the internode of a stern of Zea Mais, a, King 

 of an annular tracheid ; sp, spiral tracheid ; n and m', vessels with bordered pits ; v, sieve- 

 tubes ; s, companion cells ; cpr, compressed protophloem ; I, intercellular passage ; vg, sheath ; 

 /, cell of fundamental tissue, (x 180.) 



phyta ; examples are afforded by some of the lied and Brown Seaweeds 

 (Rhodophyceae and Phaeophyceae). In the Laminariaceae these con- 

 ducting tracts contain elements which closely resemble sieve-tubes ( 108 ). 

 The thallus of some Liverworts is traversed by a strand which 

 resembles the nerve of a leaf. Bundles sharply limited from the 

 surrounding tissues first appear in the Mosses ; they occur commonly 

 in the leaves, less often in the stems. A fairly simple example of 

 this kind of conducting bundle is that of the stem of Mnium 

 which is represented in tranverse section in Fig. 161. 



