172 STRUCTURAL BOTANY 



conducts to the growing organs and reserve-stores, the 

 food which has been assimilated in the leaves, and es- 

 pecially that part of it which contains nitrogen. On 

 the other hand, the carbohydrate foods produced by the 

 leaf, such as sugar, are conveyed through the thin- 

 walled parenchyma which accompanies the bundles. 

 Both in Monocotyledons and in Dicotyledons all these 

 tissues are continuous throughout the central cylinder 

 of the stem and its branches, and ultimately with the 

 corresponding tissues of the root. Neither as regards 

 structure nor function can we point to any profound 

 difference between the two Classes, so far as the 

 primary tissues are concerned. The real distinction 

 lies in the fact that in typical Monocotyledons there 

 are no secondary tissues', the entire structure is 

 developed from the growing points once for all, and 

 henceforth no further additions are ever made, how- 

 ever long the plant may live. Consequently, we 

 generally find that the primary structure of stem 

 and root is more complex in Monocotyledons than in 

 the other Class, because in the former the primary 

 tissues have to do all the work, while in the Di- 

 cotyledons they are supplemented, or even entirely 

 superseded, by the secondary tissues formed from the 

 cambium. The few Monocotyledons which have a 

 kind of cambium are here left out of consideration. 



This difficulty, however, arises : many Monocoty- 

 ledons, when mature, have a stem which is of great 

 thickness compared with that of the same plant when 

 a seedling. Take the Maize, for instance : a well-grown 

 plant may have a stem two inches in diameter, while 



