STEMS 251 



It is a matter of common observation that the rich display 

 of spring flowers occurs in forests and wooded glens before 

 the trees come into full foliage. The working season of 

 these spring plants is between the beginning of the growing 

 season and the full forest foliage, and the subterranean 

 shoots enable them to send up their aerial parts with great 

 rapidity. After the forest leaves are fully developed, the 

 available light for work beneath the forest crown diminishes, 

 the spring flowers disappear, and the short period of activity 

 does not return until the next season. 



Another situation in which speed of development is of 

 great advantage to the plant is in regions in which there is 

 a short rainy season and a long dry season. In such regions 

 the annuals spring up with remarkable rapidity, and in most 

 cases this is made possible by food storage in underground 

 stems. 



137. Summary. The most important fact about a stem 

 is that it produces and displays leaves. The nodes not only 

 produce leaves, but also branches, and if the conditions favor 

 it, they can produce roots also ; therefore it is possible to 

 reproduce a whole plant from a node, a fact that is taken 

 advantage of in the propagation of many plants. 



The vascular system forms a hollow cylinder in the stems 

 of Gymnosperms and Dicotyledons, a cylinder that incloses 

 pith and is surrounded by cortex. The two regions of the 

 vascular cylinder are xylem and phloem, and in Gymno- 

 sperms and Dicotyledons the xylem is next to the pith and 

 the phloem next to the cortex. A notable feature of such 

 a stem is the presence of the cambium between the phloem 

 and xylem, which adds new elements to each region. The 

 additions to the xylem in the case of perennial stems (not- 

 ably trees) result in an annual increase of wood, which 

 usually appears as annual rings. This yearly increase in 

 the capacity for carrying water is associated with the pos- 

 sibility of an annual increase of branches and leaves. 



