122 



BOTANY 



less deeply they penetrate the tissues on either side of the cambium. 

 The primary medullary rays are therefore often distinguished as 

 long, the secondary as short medullary rays. The expression trans- 

 verse parenchyma is also sometimes used to designate the medullary 



£> - s & 



Fig. 136.— Transverse section of a stem of Aristolochm Sipho in the first year of its growth, show- 

 ing a vascular bundle with cambium in active division, p, Vascular parenchyma ; vlp, proto- 

 xylem ; m' and m", vessels with bordered pits ; ic, interfascicular cambium in continuation 

 with the fascicular cambium ; v, sieve-tubes ; cbp, protophloem ; pc, pericycle ; sk, inner part 

 of ring of sclerenchymatous fibres. ( x 130.) 



rays, which in fact are composed almost exclusively of parenchymat- 

 ous tissue. The cells given off by the initial layer of the cambium 

 for the formation of medullary rays do not undergo a further division 

 as in other cases, but assume at once the character of medullary ray 

 cells. 



The cambium cells have, for the most part, the shape of right-angled prisms, of 

 which the radial diameter is smaller than the tangential. The ends of these prisms 



