MORPHOLOGY 127 



In the central cylinder of roots, the radially arranged strands of 

 xylem and phloem pursue a straight course parallel to one another. 

 The xylem and phloem strands of lateral roots are inserted upon the 

 corresponding strands of the parent root. Those of adventitious 

 roots are attached to the corresponding tissues of the stem or root 

 from which the adventitious root arises. The strands of xylem and 

 phloem of the main root of a phanerogamic seedling are continuous 

 with the leaf-trace bundles in the hypocotyl ( 12 ). 



Increase in Thickness by Continued Enlargement of the Primary 

 Meristem. The growing points of those Monocotyledons which 

 have large stems (Palms, Pandanaceae, some Liliiflorae) exhibit this 

 feature. The growing point thus attains a considerable thickness, and 

 the stem has from an early period its definite circumference. The 

 increase in number of cells takes place in a zone lying at the 

 periphery of the growing point ( m ). 



A Special Form of Growth in Thickness of the Stem by means 

 of the Continued Enlargement of the Fundamental Tissue. This is 

 often exhibited by Palms, and leads in them to the formation of stems 

 which may be more than 1 metre in diameter. EICHLER ( 122 ) has 

 shown that 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 thickening of the walls, the strands of sclerenchy- 

 matous fibres accompanying the vascular bundles on their phloem 

 sides also become greatly enlarged. In this form of growth in 

 thickness no new elements are formed. 



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 

 rnay develop in this way. 



The Secondary Tissues ( 123 ) 



After growth in length is finished through the activity of a 

 cambial tissue, functioning either as a primary or secondary meristem 

 (p. 99), 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 



