THE ANGIOSPERMAE : STEMS 



903 



cells known. They may persist for an indefinite period without growth 

 and without division. 



Medullary rays in woody Dicotyledons have been classified into a number 

 of types on the following characters : (i) Whether both uniseriate and 

 multiseriate rays are present, or only one kind. (2) Whether the cells com- 

 posing the rays are all alike short, or whether some are vertically elongated. 

 The heterogeneous type, with both kinds of rays and both kinds of cells, is 

 regarded as the most primitive, and the homogeneous type with only 

 multiseriate rays of short cells is probably the most advanced. 



Where the xylem ray cells abut on vessels, the ray field, i.e., the area of 

 contact between ray cell and vessel, has simple, not bordered pits, which 

 are sometimes single and ver}' large, but more often small and either circular 

 or elliptical (Fig. 886). 



Medullary ray cells 

 Xylem vessel 



Pit 



Fig. 886. — Acer pseiidoplotatius. Longitudinal 

 radial section showing a medullary ray 

 crossing the xylem elements. In some 

 of the ray fields circular pits may be seen. 



II. Types of Wood. At the resumption of activity in spring the cambium 

 produces comparatively large, thin-walled, rapidly expanding xylem cells, 

 the spring wood, which meets the need for active conduction of water to the 

 developing buds. At the height of summer cambial growth is reduced, but 

 it again increases to a secondary- maximum in autumn, associated with in- 

 creased rainfall and an abundance of photosynthetic carbohydrates. The 

 autumn wood is, however, much denser than the spring wood and contains 

 a higher percentage of fibres. It is the dense zones of autumn wood which 

 mark the " grain " of commercial timbers. 



Tracheids, vessels and fibres may be formed at any time during the year's 



