SECONDARY INCREASE IN THICKNESS OF STEMS AND ROOTS. 



T29 



into permanent tissue, and indeed into thick-walled parenchyma, which now forms 

 the secondary fundamental tissue between the secondary fibro- vascular bundles. 

 Since the cells of the thickening-ring which face inwards pass over in centrifugal 

 succession into permanent tissue, while the outermost divide repeatedly, the whole 

 ring continually moves centrifugally as it increases in diameter, and leaves behind 

 new bundles and parenchymatous cells. In Fucca Millardet found the origin of 

 the ring of meristem (thickening-ring) as litde as 3 mm. below the apex of the 

 stem ; in Calodracon {Cordyline) Jacquini, the meristem-ring is derived immediately, 

 according to Nageli, from the primary meristem of the apex of the stem, this layer 

 remaining in a condition capable of 

 division while the primary vascular bun- 

 dles and fundamental tissue are being 

 differentiated out of the primary meri- 

 stem. 



(2) The Type of tiormal Gymnosperms 

 and Dicotyledons may be made clear by 

 a reference to Fig. 105, which — with 

 the exceptions of a few points of sub- 

 ordinate importance, such as the substi- 

 tution of six fibro-vascular bundles for 

 eight — represents in a simple diagram- 

 matic manner the phenomena connected 

 with the growth in thickness of the 

 hypocotyledonary portion of the stem 

 (tigellum) of Ricinus conununis. We 

 may commence with the period when, in 

 the seedling stem, the fibro-vascular bun- 

 dles — which are prolongations down- 

 wards of those bundles which bend out- 

 wards above into the first leaves or 

 cotyledons — have become clearly differ- 

 entiated. They lie, when seen in trans- 

 verse section (Fig. 105 ^4), in a ring, and 

 run parallel to one another and to the 

 surface of the stem. The ring of fibro- 

 vascular bundles divides the primary 

 fundamental tissue into pith {M) and 

 cortex {R\ which, however, still retain their connection by broad bands of funda- 

 mental tissue lying between the bundles, the Medullary Rays. Each of the bundles 

 consists of an outer phloem-portion (/>) add an inner xylem-portion {x\ between 

 which lies a layer of cambium. The next 'change consists in the bands of cam- 

 bium belonging to the bundles uniting into a' continuous ring (Fig. 105, B\ 

 meristem being formed between each pair of adjacent bundles by divisions in 

 the corresponding layer of the medullary rays, as is more exactly shown in Fig. 93 

 (p. 112), which relates to this stage of development. Although there is no essential 

 difference between this portion of the cambium-ring and that which lies in the 



K 



FIG. 105.— Diagrammatic representation of the secondary increase 

 in thickness of a normal dicotyledonous stem. 



