336 PRIMARY ARRANGEMENT OF TISSUES. 



with secondary growth in thickness, this border-zone maintains its capacity for 

 division, and becomes a portion, or a starting-point, of the cambial ring (Chap. XIV). 

 In other bundles the cessation of the divisions takes place early in each transverse 

 portion, simultaneously with the general differentiation of the tissues ; and according 

 to the degree in which this happens, the radially arranged boundary layer becomes 

 less clear. It is therefore also, especially in Monocotyledons, the less possible 

 everywhere to fix a sharp boundary between phloem and xylem, the less numerous 

 and crowded the lignified elements in the latter are. 



The word Cambiform used above was first adopted by Nageli (Beitr. I. p. 4). For the 

 meristematic strand giving rise to a vascular bundle, and consisting of thin-walled longi- 

 tudinally extended cells in which longitudinal division goes on for some time, Nageli 

 uses the traditional and certainly ambiguous name Cambium, for which Sachs has re- 

 cently substituted Procambium. The tissue of the phloem which has arisen from this 

 cambium and passed over into the permanent condition is called by Nageli in its totality 

 the Cambiform, i. e. cambium-like tissue, as its elements are so similar to that of the 

 cambium, in their elongated form and thinness of wall, that former observers have 

 actually identified them with it. Our present knowledge imposes the necessity of severing 

 the sieve-tubes from Nageli's Cambiform, as a distinct kind of tissue. The name there- 

 fore, once being in existence, remains reserved for their characteristic companions, and 

 may the better be used for them as it is for the most part literally applicable to them, 

 even when the original meaning, indicated in Chap. XIV, and differing from that above- 

 mentioned, is restored to the word cambium. 



The structure of the phloem of collateral vascular bundles of the main stems has been 

 described in the preceding paragraphs; the description was based primarily on the 

 numerous investigations now existing of siich objects as afford a clear and certain 

 insight, owing to the size of the elements in question. It was already mentioned under 

 2 and 3, that in certain cases differences occur from the type described under i, and that 

 doubts exist as to the structure. Besides these definitely characterised cases, there are 

 many others, especially concerning the smaller vascular bundles, in which we know 

 nothing more of the structure of the phloem than that it consists of thin-walled, narrow, 

 and elongated elements, the delicacy and smallness of which, as well as the tendency of 

 their walls to swell, which interferes with their preparation, is deterrent to accurate 

 , research. Where these difficulties have already been successfully overcome, the struc- 

 ture described has always manifested itself. An uninterrupted series of transitions 

 leads from the cases with relatively large, easily intelligible elements, to those where 

 they are most delicate and difficult. No grounds whatever exist which would compel 

 us to assume an essentially different structure. I therefore think that I am right in 

 stating that the structure described exists in all vascular bundle-trunks here in question, 

 and the more so as it is really not long since the largest sieve-tubes were first clearly 

 recognised, as remarked at p. 182, and I do not doubt that further investigations, which 

 are in any case a necessity, will justify what has been said. 



In the xylem of many collateral bundles an intercellular passage occurs, which 

 follows the course of the whole bundle, sometimes next to or within the otherwise 

 persistent xylem, sometimes so that, though the latter is originally formed, the 

 trachese become destroyed and degraded as the parts expand. 



In numerous Monocotyledons, the Equiseta, and some Dicotyledonous water- 

 plants, at the inner edge of the bundle, where the primitive trachese are placed, a 

 passage is formed by the peripheral extension of the surrounding cells, i.e. schizogene- 

 tically (p. 200), while the external part of the xylem attains perfect development and 

 is persistent. This severing of the original continuity of tissue goes on within the 



