6o 
Lee.— The Morphology of Leaf-fall. 
Class II (a). Tilia europaea, Linn. 
We now enter upon a description of a second type of leaf-fail, taking as 
our first example the common Lime. The sequence of events is in many 
respects very similar to that of the examples just described for Class I, but 
there is one essential difference. Here again, taking the class as a whole, 
ligno-suberization may or may not occur previous to defoliation ; but in 
every case without exception, new walls running in all directions are pro- 
duced in the cells of the Protective-layer before ligno-suberization takes 
place. This is an essential distinction independent of external conditions. 
In the present species the petiole is a relatively long, slender, wiry 
organ which gives place above to the broad lamina, spreading a little 
towards the base where it 
curves sharply to join the 
stem. In the curve is firmly 
wedged the large asymmetrical 
axillary bud, which appears 
slightly to displace the base of 
the petiole. A slight furrow 
marks the junction of stem and 
petiole. 
In the upper part of the 
petiole a conspicuous sheath 
of stereome surrounds the ring 
of vascular bundles. As the 
leaf-base is approached the 
vascular cylinder gives place to 
three separate bundles ; at the 
same time there is a gradual 
reduction in the quantity of 
sclerenchyma (Text-fig. 6), and the amount of lignification is correspondingly 
reduced. At the leaf-base itself there is practically no lignification outside 
the vascular bundles, and the stereome accompanying each has almost 
disappeared. On joining the vascular ring in the stem the sclerenchy- 
matous patches outside the phloem have again increased to their normal 
amount and are completely lignified. 
There is little or no difference in size between the cells ot cortex and 
petiole, but the junction between the two is marked by a layer of much 
smaller cells. Early in the season starch granules are present in the cortex, 
but are entirely absent from the petiole with the exception of the bundle 
sheath. Simple and compound crystals of calcium oxalate are common to 
the parenchyma of both organs, and secreting cavities of various size are 
Text-fig. 6. Tilia europaea. Longitudinal section 
of leaf-base just previous to leaf-fall. 
