DEVELOPMENT OF PLANTS 



36s 



forests are rapidly disappearing, because with few exceptions the 

 trees are able only to develop from seeds. Possibly the rapid 

 covering of living cells of the stump by the resinous substances 

 excludes the atmosphere so effectually as to prevent any further 

 growth (page 75). This resinous liquid is obtained in large 

 quantities from certain species of pine by removing the bark and 

 a little of the wood from the side of the trees when the liquid 

 exudes and is caught in a pan placed at the bottom of the cut 

 surface or in a pocket made in the tree. This substance is re- 

 moved from time to time and distilled, yielding turpentine and 

 resin. Balsam is a resin obtained from the balsam fir {Abies), 

 (b) The Sporangia of the Finales. — ^The sporangia resemble 

 those of the cycads in origin and structure and are developed 

 upon sporophylls that are usually grouped in strobili (Figs. 251, 



Fig. 251. Microsporophylls of the pine: I, branch of pitch pine with 

 leaves spirally arranged in fascicles of three and also with several strobili, 

 St. At tip of branch the bud, b, is shown elongating and bearing numerous 

 fascicles of leaves that are still enclosed by paper bracts. 2, surface and side 

 view of microsporophyll, showing arrangement of sporangia, sp. 2a, micro- 

 sporophyll of juniper bearing three sporangia. — H. O. Hanson. 



252). The microsporangia are formed on rather small, delicate 

 and often brightly colored sporophylls that perish as soon as the 

 spores are scattered (Fig. 251, 2). The microsporophylls bear 

 one or several sporangia on their under surface and are hygro- 



