SECONDARY PHLOEM 655 



the smaller medullary rays iu Cytisus Laburnum. Here certain 

 elements of the cambium first undergo one or two preliminary divisions. 

 Every primordial mother-cell of a medullary ray becomes divided by a 

 radial longitudinal wall. One of the two daughter-cells may thereupon 

 be directly transformed into a definitive medullary ray mother-cell ; it 

 may, on the other hand, form a curved longitudinal wall, and thus cut 

 off a small spindle-shaped cell, which then gives rise, by transverse 

 division, to the initials of the several cell-rows of the medullary ray. 



Medullary-ray initials behave essentially in the same way as 

 ordinary cambial cells ; by means of successive tangential divisions 

 they add new elements, on their outer side, to the phloem-ray, and on 

 their inner to the xylem-ray. Krabbe has shown, for a number of 

 woody plants, that the daughter-cells of the ray-initials never divide 

 any more, but become directly converted into permanent tissue- 

 elements, which accrue to the xylem-ray, or to the phloem-ray, according 

 as they are cut off on the inner, or on the outer side. In an eight- 

 year old branch of Cytisus Laburnum, examined by the author, a 

 particular ray-initial was found to have cut off two phloem-ray and 

 eight xylem-ray elements in the course of each period of vegetation. 

 If the duration of the latter be taken as approximately equal to 200 

 days, it is clear that successive divisions of the ray-initial must have 

 taken place, on an average, at intervals of twenty days. 



Translocation proceeds most actively in spring and in autumn ; 

 at these times the ray-initials are traversed by a constant stream 

 of migrating carbohydrates, and thus for the time being act 

 principally as conducting elements. In some woody stems indeed 

 {Que reus pedimculata, Feigns sylvedica, Prunus Cemsus, and especially 

 Cytisus Laburnum), the change of function is so complete as to involve 

 certain histological changes. Almost all the ray-initials soon lose their 

 meristematic character and acquire thickened and pitted cell-walls; in 

 fact, they acquire all the characteristics of typical phloem-ray cells 

 (Fig. 274, m). In such cases as these, therefore, the xylem- and phloem- 

 rays are directly continuous with one another from late autumn to early 

 spring ; when active growth in thickness is resumed, new ray-initials 

 are formed by division of the innermost elements of the phloem-rays. 

 Here, therefore, the cambial initials constitute a secondary meristem 

 which is regenerated every year at a number of points. 



B. THE SECONDARY PHLOEM (EXTRA-CAMBIAL SECONDARY TISSUE). 350 



The extra-cambial products of the cambium are frequently included 

 in the topographical definition of " bast " ; naturally this usage cannot 

 be accepted in the present work, since the term " bast " has already 

 been restricted to a particular constituent of the mechanical system. 



