CHAPTER VIII 



cylinders of wood and bark is the cambium layer, the layer of new 

 cells, which during the growing season multiply very rapidly, becom- 

 ing new wood and new bark. It is the delicacy of these new cells in 

 spring time that makes it possible to slip the cylinder of bark off the 

 wood of willow and elder twigs. Early in the growing season the 

 wood cells have comparatively thin walls and long diameters, later 

 the tissue is much more compact ; this gives rise to the rings of 

 annual growth. 



The coverings of buds, the disposition of leaves in the bud, and the 

 devices for protecting the young leaves as the buds develop into 

 shoots, are interesting topics. The nature of the wrappings can be 

 made out by the older pupils. That the scales are often modified 

 leaves, is clearly shown in developing walnut buds, there being many 

 intermediate forms between the simple scale and the compound leaf. 

 Many of our trees use stipules for bud wrappings, not only the fig, 

 India rubber and magnolia, but the apple, cherry and their kind, and 

 the oak, alder and others. In the sycamore the collar-like structure is a 

 pair of united stipules, but in this case the stipules become a per- 

 manent part of the foliage. Leaves in the bud have their conductive 

 tissue very prominent, but the parenchyma is not well developed, and 

 there is little or no cuticle on the epidermal cells. As the leaves 

 throw off their wrappings the delicate parenchyma cells are in special 

 need of protection against over-transpiration, and the same devices 

 that serve this purpose, also protect against cold and moisture. It is 

 not against frost chiefly that protection is needed ; leaves need to 

 retain at night, as far as possible, the heat acquired during the day, 

 since growth goes on night and day. The various protective devices 

 are easily seen. Often the soft tissue is plaited, folded or crumpled 

 in, the veins only being exposed. Usually the young leaves assume 

 a vertical position ; besides this, there are coverings of varnish, or 

 woolly or silky hairs. Sometimes there are membranous stipules 

 that grow and continue to screen the young leaves until they have 

 attained considerable size, as in the magnolia and the India rubber. 

 The use of the red color to young leaves has already been noticed in 

 Chapters III and V. This color, and the vertical position, seem to be 

 all the protection the young leaves of the Eucalyptus and the pepper 

 require. 



The willow in mind when this chapter was written, was Salix 

 lasiolepis, Benth., an early flowering species common throughout the 

 state ; other species might differ in detail. The pistil of the willow, 

 when the fruit is mature, splits into its two carpels. The fluff attached 

 to the seeds floats them on water as well as in the air. Another aid 



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