138 BOTANY part i 



by Palms and Pandanaceae and leads in them to the formation of 

 stems which may be more than 1 metre in diameter. The 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 thicken- 

 ing of the walls, the strands of sclerenchymatous fibres accompanying 

 the vascular bundles on their phloem sides also become greatly en- 

 larged. In order to maintain the power of conduction of the existing 

 vascular bundles these are reinforced by the development of new connec- 

 tions in the periphery of the stem ('"*^). 



Enlargement due to Continued Cell Divisions in the Fundamental 

 Tissue. — This is seen most strikingly in the growth of the fruits of 

 many Angiosperms. In their development from the small rudiment 

 in the flower there may be a great increase in size, although no new 

 meristematic tissue has been formed. The growth is due to repeated 

 divisions in the epidermal cells and in those of the fundamental tissue. 

 The tissue thus produced may when mature be thin-walled, or its cells 

 may undergo various changes in form and in the thickness of their 

 walls. From small structures such large fruits as those of the Gourds 

 may develop in this way. ^ 



B. The Secondary Tissues (^^") 



After growth in length is finished through the activity of a 

 cambial tissue, functioning either as a primary or secondary meristem 

 (cf. p. 107), secondary tissues are added to the previously existing 

 primary tissues, or even substituted for them. Although phylo- 

 genetically considered, secondary tissues seem to have been developed 

 first in the Pteridophytes in forms now only known in a fossil 

 condition (Calamarieae, Sigillarieae, Lepidodendreae), they first became 

 of general occurrence in the Phanerogams. In them the formation 

 of secondary tissues is almost exclusively confined to the roots and 

 stems ; secondary growth is met Avith in some foliar structures, biit 

 only in a slight form. 



Cambium Q^^). — The primary and secondary meristenis, the 

 activity of Avhich gives rise to secondary growth, are given the name 

 of Cambium. These tissues persist in a meristematic condition and 

 undergo successive divisions by walls parallel to one another, so that 

 the cells produced from them are arranged in radial rows. A similar 

 mode of division characterises the primary meristem in the pro- 

 cambial strands, which give rise to collateral vascular bundles, and 

 this might also come under the term cambium. It is advisable, how- 

 ever, to restrict this term to meristenis giving rise to secondary 

 growth. As a rule the cambium forms new tissue-elements on both 

 sides, but cases are not wanting in which this takes place to one 

 side only. 



