COLLATERAL BUNDLES 



355 



by a group of narrow vessels, the latter alternative being exemplified by 

 many Grasses. In ceitain other Monocotyledons (e.g. Mum, Maranta, 

 Calamus [Fig. 144]), the vascular bundles are characterised by the 

 presence of a single very wide vessel in the centre of each hadrome- 

 strand. According to Scherer, the development of isolated vessels of 



Fio. 143. 



Collateral bundle, with partial fibrous sheaths, from the haulm of Bambusa Stmonii. 

 g, large pitted vessel ; I, intercellular passage [which has taken the place of the proto- 

 hadrome]. The space between g and I is occupied by smaller vessels and tracheides. 

 s, sieve-tubes, accompanied by companion-cells and phloem-parenchyma (cambiform 

 cells) ; b, the four partial fibrous sheaths ; of these the outer shelters the leptome 

 strand, while the two lateral ones protect the large pitted vessel ; p, thick-walled 

 parenchymatous ground-tissue. 



exceptional width which usually takes place immediately after the forma- 

 tion of a few narrow [protoxylem] elements, is possibly connected with 

 the general mode of life of the plants in question. For the most part the 

 species provided with this type of bundle are bulbous or tuberous forms, 

 which develop their vegetative organs very rapidly. Their transpiratory 

 activity, and hence the demands made upon their water-conducting 

 system, are thus liable to increase quite suddenly. It is this sudden 

 demand that is provided for by the formation of the aforesaid large 



