142 BOTANY PART i 



tissues corresponding to the needs of the growing plant. This is 

 effected by a continued process of cell division forming secondary 

 tissues and leading to a SECONDARY GROWTH IN THICKNESS of the 

 stem and roots. Secondary tissues are those that are added to or 

 replace the primary tissues as a result of the activity of a secondary 

 meristem or CAMBIUM (cf. p. 47). Such secondary growth occurs 

 in herbaceous as well as in woody plants. 



To the FIRST TYPE ( 76 ) belong the mostly herbaceous Pteri- 

 dophytes and Monocotyledons, including nearly all the forms that 

 have definite stems (Tree-ferns, Palms, Pandanaceae, certain Lilii- 

 florae). Thus in these stem-forming Monocotyledons the embryonic 

 stem remains very short on germination. The primary meristem of 

 the flattened growing point increases in breadth, leading to the axis 

 of the seedling from which the stem will continue having a consider- 

 able thickness from an early stage. 



In such forms as the Palms and Pandanaceae the stem may continue to 

 increase slightly in thickness after the permanent tissues have developed by a 

 process of expansion of the cells. The cells of the sclerenchymatous strands 

 which accompany the phloem of the vascular bundles may thus increase in 

 diameter leading to an enlargement of the strand as a whole. In places this 

 growth in thickness may be accompanied by divisions in parenchymatous cells 

 (e.g. in some Palms). 



The majority of herbaceous and woody Gymnosperms and Dicoty- 

 ledons and some arborescent Liliiflorae belong to the SECOND TYPE C 77 ). 

 The primary thickening or maturing of the stem and root dependent 

 on the enlargement of cells is in them followed by increase in number 

 of the cells in a special meristematic zone, the cambial ring. 



The secondary thickening in annual, scrambling, and twining plants often only 

 begins in older internodes which have long attained their full primary size. In 

 the twigs of trees, on the other hand, the secondary growth may start early, even 

 before the primary tissues are fully developed. 



Secondary growth in thickness was present in certain Pteridophytes known 

 to us as fossil remains, but only became of general occurrence in the Gymnosperms 

 and Dicotyledons. 



Secondary Growth in Thickness of Monocotyledons. In some 

 arborescent Liliiflorae (Dracaena, Cordyline, Yucca, Aloe) the axis 

 exhibits growth in thickness due to a secondary meristem. This 

 arises in the cortex where it abuts on the central cylinder in which 

 the vascular bundles are scattered in the manner characteristic of 

 Monocotyledons. In transverse sections divisions can be seen to 

 begin in an annular zone of mature cortical cells. In Dracaena this 

 happens at a considerable distance from the growing point, but in 

 other cases it may start close to it. A cylindrical meristematic zone 

 a number of cells deep is thus formed ; the cells are prismatic and fit 

 together without intercellular spaces. As a result of the formation of 



