452 THE BIOLOGY OF FLOWERING PLANTS 



by the formation of a special abcission layer of cells weeks 

 before the event, though the actual separation from the 

 tree may be hastened by frost, or may be induced earlier 

 in the season by exceptionally dry and hot weather. Nor is 

 it usually possible to cause the resting buds to open in winter 

 by bringing branches or potted plants into favourable 

 hothouse conditions ; growth does not even start till far 

 on in winter, any more than the buds begin to swell after a 

 succession of exceptionally mild November or December 

 days outside. 



There are very great differences in the behaviour of 

 various trees, shrubs, and herbs. Thus for woodland 

 herbaceous perennials, which naturally come up in spring, 

 Diels (191 8) has shown that some, such as Asperula odorata 

 and Merciirialis peretmis, can be forced to renewed growth 

 at any time of the year by suitable temperature ; others, 

 such as Leucojum vernum and Arum maculatum, can be forced 

 in autumn, but not through the summer ; a third type, 

 Corydalis solida and Polygonatum multiflorum, can be forced 

 only in spring. Such shrubs as the roses and the brambles 

 are capable of continued growth throughout the year, 

 whenever the temperature is favourable, but most of our 

 shrubs and trees form resting buds earlier or later in the 

 summer, and these cannot be forced to open by raising the 

 temperature in winter. 



The actual mode of growth through the summer is very 

 different in different trees. In the beech and the oak, after 

 the buds open in May and the leaves and the internodes 

 laid down in them have fully expanded, new buds are 

 formed which, by the end of June, contain the leaf rudiments 

 for the following year. No further growth takes place, 

 with this exception, that a few of the terminal buds open in 

 July and produce the " Lammas shoots," which stand out 

 in the pale green of youth against the darker mature leaves, 

 in August. In the elders, maples, and poplars many shoots, 

 in the hazel the basal shoots, show continued growth till 

 late autumn ; in the alder and elm growth goes on through 

 the summer. This summer growth is not necessarily 



