SECONDARY INCREASE IN THICKNESS OF STEMS AND ROOTS. 129 
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 Fucea Millardet found the origin of 
the ring of meristem (thickening-ring) as little 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 normal 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 co?nmunis. 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 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 (yT/) and 
cortex (i?), 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 (/>) and an inner xylem-portion {x), between 
which lies a layer of cambium. The next change consists in the bands of cam- 
FIG. 105.— Diag-rammatic representation of the secondary increase 
in thickness of a normal dicotyledonous stem. 
bium belonging to the bundles uniting into a continuous 
(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 Ues in the 
K 
