DEVELOPMENT OF THE VASCULAR BUNDLES. 389 



Sachs ^ has introduced the name Procambium for the former, because the term 

 cambium, if applied to them, would be ambiguous, meaning two different things, 

 namely, on the one hand the strands in question, and on the other the initial layer 

 of secondary growth, which has been traditionally designated by this name, and of 

 which Chap. XIV will treat. We here entirely pass over the use of the word cambium 

 mentioned at p. 4, introduced by some authors for all those groups of cells which 

 are called Meristem in this book. Finally, Russow'^ call the strands in question 

 Desmogen. 



It is essentially a matter of indifference which name is used, provided it be known 

 what it means ; that is, in the present case, that we are dealing with groups of cells 

 which fall under that general conception of Meristem — gradually passing over into 

 differentiated tissue — to which we have adhered in this book, and that we are 

 dealing with strands which are different from the cambium to be treated of in 

 Chap. XIV, although they may actually stand in the closest anatomical and genetic 

 relation with it. It may well be advisable, however, to avoid any term which would 

 recall a different idea, or which would seem to imply more than the simple fact that 

 the strands in question are the young vascular bundles, i. e. the beginnings of the 

 latter ; and for this reason we may apply to them the term initial strands or initial 

 bundles, in agreement with the other terminology used in this book. The initial 

 strands arise from and consist of initial cells, of successively different degree and 

 value, from which the elements of the vascular bundle are derived. 



In typical roots the axial meristematic strand, termed the plerome in the Introduc- 

 tion, is the initial strand for the vascular bundle. Its differentiation from the surround- 

 ing meristematic layers, the degree of completeness of which differs according to the 

 particular types, has been described above in the Introduction ; in the case of lateral 

 roots arising on a main root, this will still have to be discussed below. Here we 

 have only to repeat that the individual parts of the vascular bundle are already at an 

 early stage to be distinguished as special layers of the initial strand. In Fig. i86,' 

 reproduced here, and in Figs. 2 and 4-6, above, the layer or row marked p and pc, 

 the same which in Fig. 3 is marked/" next to x—x, is the pericambial layer, which 

 may be followed up to the common initial cells lying at the apex of the plerome 

 body ; in the same figures v or g marks in each case an initial row for a vessel or 

 row of tracheides, and of this initial row the same holds good as of the pericambium. 

 In the case of the root-bundle of the Ferns and Azollas, similar conditions to those 

 in the Phanerogams may be demonstrated with still greater sharpness, as shown by 

 Nageli, Leitgeb, and Strasburger. 



In stems with a simple axial kindle the primary plerome body, like that in the 

 root, has the significance of an initial strand (Fig. i, p. 8). 



In stems with a complex bundle-systern, and in leaves, the conditions are different. 

 We will first consider simply the earliest appearance of the vascular bundle, without 

 reference to the question as to its special place of origin. The development begins 

 with the fact that a single row, or more frequently a strand consisting of several rows, 

 of the originally uniform primary meristematic cells, lying in a position correspond- 

 ing to the course of the bundle, undergoes divisions by means of longitudinal walls, 



' Textbook, 2nd Eng. ed. p. no. ^ Vergl. Unters. p. 178. 



