120 BOTANY 



inner sides become turned outwards. The separation of the xylem 

 and phloem may be accomplished without any further division of the 

 bundles, the xylem and phloem portions of which then simply arrange 

 themselves side by side ; or it may be accompanied by a complete 

 radial division of the phloem, and a subsequent coalescence between 

 the parts of the phloem of different bundles. In the adjoining figure 

 (Fig. 134) the transition stages occurring in the Yew (Taxus baccata) 

 are diagrammatically shown. The two vascular bundles from the 

 cotyledons (cc), in their passage through the hypocotyl, undergo a 

 radial division extending through the phloem to the protoxylem. 

 The two halves of each xylem portion separate from one another, 

 and the protoxylem strands turn through 180° and thus come to lie 

 on the inner side of the xylem strands. The two halves of the phloem 

 portion separate from each other in a tangential direction, and 

 coalesce with the phloem portion of the adjacent bundle. Thus, in the 

 root, two phloem strands finally alternate with two xylem strands. 

 At the same time, owing to the disappearance of the pith, there is a 

 diminution in the diameter of the central cylinder of the roots. 



A Special Form of Growth in Thickness of the Stem by means 

 of the Continued Enlargement of the Fundamental Tissue. — This 

 is often exhibited by many Palms. Eichler has shown that growth 

 in thickness is solely due to the continued expansion of the already 

 existing cells of the fundamental tissue of the central cylinder. In 

 this process, by the expansion of the cell lumen and increased 

 thickening of the walls, the strands of sclerenchymatous fibres accom- 

 panying the vascular bundles on their phloem sides also become 

 greatly enlarged. In this form of growth in thickness, which appears 

 to be limited to the Palms, no new elements are formed. 



The Secondary Tissues 



Through the activity of a cambial tissue, functioning either as a 

 primary or secondary meristem (p. 90), secondary tissues are added 

 to the previously existing primary tissues, or even substituted for 

 them. Although, phylogenetically considered, secondary tissues seem 

 to have been developed first in the Pteridophytes in forms now only 

 known in a fossil condition, Calamaricae, Sigilhrieae, Lepidodendra, they 

 are now only of general occurrence in the Phanerogams, and in them 

 the formation of secondary tissues is almost exclusively confined to 

 the roots and stems. 



The Cambium Ring. — The cambium of the open vascular bundles of 

 Gymnosperms and Dicotyledons, which exhibit a growth in thickness 

 commences its activity almost directly after the formation of the 

 primary tissue. The cambium or primary meristem remaining between 

 the xylem and phloem portions of the bundles consists of only a few 



