Growth. 23 



torn by the increase in volume of the tissues formed from the 

 generative layer, or cambium. The concentric rings of wood 

 in the stem may be made out with the unaided eye, if the 

 surface is fairly smooth. One ring is ordinarily formed every 

 year. If the growing season should be interrupted by a mid- 

 summer drought, two rings might be formed in a single 

 season. 



The thin cylindrical sheet of cambium develops cells in its 

 inner layers, which become converted into woody tissues. The 

 tissues external to the cambium arise by various methods. 

 (For a detailed study of the characters of a woody stem see 

 Gregory, E. L., " Elements of Plant Anatomy.") 



Examine the trunk of any tree which has been recently 

 felled, or the freshly cut stump, and count the number of rings 

 of wood. Taking into account the exigencies of climate, the 

 age of the tree in years will be something less than the number 

 of annual rings. Determine the height of the tree. If possi- 

 ble, compare the two measurements with those obtained from 

 a second tree of the same species. 



8. Growth of leaves with netted veins. Select 

 some plant, such as sunflower (ffelianthus) , dandelion (Tarax- 

 acum ) , peach (Amygdalus) , maple (Acer) , chestnut ( Castanea) , 

 or squash (Cucurbitd), in which the young lef as it unfolds 

 from the bud is but a fraction of its adult size. Mark off a 

 series of intervals of 5 mm. in a straight line from the base to 

 the apex, using a glass tube drawn to a fine point as a pen. 

 Mark off a second series of right angles to this in the middle 

 of the leaf. Take lengths of these intervals on successive days 

 until full size has been reached. What is the position of the 

 zone of maximum elongation ? Does the extension of a leaf in 

 breadth come from the growth of tissues near the midrib or 

 near the margin? What is the daily rate of growth in both 

 directions? 



9. Growth of a frond of a fern. Select a fern with 



