THE AXGIOSPERMAE : STEMS 



841 



orders is the maximum number, so far as is known, and the majority of plants 

 have only two or three orders. Like most other classifications this must not be 

 applied too literally, for there are many cases where the distinctions are obscure. 

 Reference was made above to the dropping of the terminal portions of 

 young shoots in Lime and Elm. In Poplar and Oak a similar shedding of 

 branches, called cladoptosis, occurs on a larger scale. Numbers of small 

 branches are dropped every year and inspection shows that they are usually 



2nd year apex 



1st year apex 



Fig. 834.— P>;7« malus. A three-year-old flowering 

 spur with terminal cyme of flowers. 



those devoted to the production of male flowers. As these twigs have no 

 leaves except at the apex, they grow very little and therefore fail to maintain 

 a firm union with the growing tissues of the trunk. The outward pressure of 

 these expanding tissues soon breaks the insecure connection of the twig and 

 it falls. 



An analogous fate befalls the dead twigs which are often left on the 

 lower and inner parts of large trees. These twigs, being unfavourably placed, 

 have so little power of growth that their water supply is drained away by the 

 competitive demands of higher and more vigorous shoots. Once dead they 

 have no further power of maintaining their union with the growing tissues 

 of the parent branch, and they are broken off by its continued expansion. 

 Their stumps get buried in the wood of the growing trunk and appear as 

 knots when the wood is cut up for use. 



