Seasonal Notes— October 



27 r 



those of the hazel are still retained and show but little change 

 in colour, such is also the case with sapling oaks. The 

 bracken is quite yellow." On November 1 of that year fifty- 

 five wild flowers were on view in the Museum Vivarium. 

 In 1902 the heath dodder was not very abundant. 



It is on record that in some tropical forests, notably in 

 Brazil, the trees during the dry season are as bare as our 

 elms are in winter. 



At the time of writing (September 22), probably not so 

 many leaves are falling as were during the first week of the 

 month. The same with apples, hardly any have fallen during 

 the past week from trees in a garden from which they were 

 continually dropping a fortnight ago. Even the limes have, 

 as yet, lost but a few of their lower leaves. Dr. Anderson 

 observed that after a terrific cyclone in October, 1864, several 

 trees in the Botanic Gardens at Calcutta put forth new leaves,, 

 which remained throughout the winter. In the following 

 spring some produced flowers, others failed to do so. What 

 is the cause of the autumnal tints ? Clearly over-oxidisa- 

 tion of the leaf tissues. In autumn the vascular system of a 

 deciduous leaf slowly becomes deranged. Though carbonic 

 acid is still absorbed, oxygen is not so freely given off. It 

 accumulates owing to the blocking up of the cells at the base 

 of the leaf stalk. In excess oxygen causes the leaf to become 

 yellow and finally reddish-brown, as is well shown in beech 

 trees. If some of these coloured leaves are steeped in alkali 

 they will temporarily return to the green colour ; if im- 

 mersed too long they become brown, but will again assume 

 the green tint for a time if removed. The autumn colour 

 changes of leaves may also be assisted by chemical sub- 

 stances which the tree has absorbed through its roots. 



It must be clearly understood that exfoliation is not an 

 accidental but a vital process. Preparation for the detach- 

 ment of the leaf has been taking place some months before 

 A layer of brittle corky cells have been slowly forming at the 

 base of the leaf stalk, and another layer of delicate cells 

 22 



