SECT. I MORPHOLOGY 155 



in tangential section, and iuHuence the course of the adjacent elements of the 

 xyleni. 



Secondary Growth of Monocotyledons. — There are certain mono- 

 cotyledonous plant families and genera, especially Dracaena, Yucca, 

 Aloe, and the Dioscoreaceae, in tlie stems of which a cambium ring 

 is developed. As in such cases, the cambium ring generally arises in 

 the pericycle, outside tlie scattered vascular bundles and from the 

 fundamental tissue, it is a secondary meristem ; it does not, as in 

 Dicotyledons and Gymnosperms, produce continuously wood and bast 

 in opposite directions, but, instead, only new tissue to the inner side. 

 This later becomes differentiated into closed vascular bundles embedded 

 in a thick-walled parenchymatous ground tissue. 



The cambial ring in these cases is only active on one side Q^^), and its initial 

 cells are replaced from the cells of the ground tissue on the outer side of the 

 ring (p. 139). 



At a later period the cambial ring commences to form cells to the 

 outer side which increase the thickness of the cortex as a thin-walled, 

 parenchymatous ground tissue (Fig. 164). When the cambium has 

 thus become active on both sides it presents the appearance of a single 

 initial layer. A true secondary growth in thickness of a monocotyle- 

 donous root is only known in the genus Dracaena. The cambial ring 

 originates not in the pericycle but in the layer of cortical cells abut- 

 ting on the endodermis. Its activity resembles that of the cambium 

 in the stem {^■^^). 



As shown in the diagram (Fig. 165) the growth in thickness of 

 a Monocotyledon differs essentially from that of a Gymnosperm or 

 Dicotyledon represented diagrammatically in Fig. 150. The stem 

 of a Dracaena "on which the diagram is based is represented as 

 unbranched. Its primary central cylinder {pc) increases in diameter 

 as the stem grows in length, and thus forms an inverted cone. It is 

 surrounded by the zones of secondary growth, the amount of which 

 diminishes on passing towards the growing point. The secondary 

 growth of the stem is continuous with that of the roots. 



In other Monocotyledons which form stems that increase in thick- 

 ness either by secondary growth or by cell division and progressive 

 increase in size of pinmary tissues (cf. p. 137) the roots show no 

 increase in thickness. An annual development of additional roots 

 from the wide base of the stem takes place, as is well seen in Palms. 

 The base of the stem of such plants may sometimes be widened out 

 like a disc as in some species of Yucca, or show a tuberous SAvelling 

 as in Nolina (Pincenectitia). The roots of such Monocotyledons often 

 attain a consideralile thickness by persistent primary growth. 



Secondary Growth of Leaves (^^^) is always very slight, and is 

 confined to a few Coniferae and Dicotyledons with evergreen leaves. 

 The increase in thickness i? due to the presence of a fascicular 



