66 Mr Potter, Additional note on the thickening [Mar. 10, 



and some species of Ranunculus ; and to this short list we may 

 add some water plants, e.g. Hippuris and Myriophyllum, etc. 



The reason why so few Dicotyledons do not increase in 

 thickness may be sought for in the habit and mode of life of 

 this important series of the vegetable kingdom. Many Dicoty- 

 ledons are perennial, being trees or shrubs, and require, as they 

 grow older and larger, that additional phloem and xylem should 

 be continually formed both for purposes of nutrition and also for 

 mechanical support ; and this is accomplished by means of the 

 cambium ring which adds continually new phloem and xylem to 

 the vascular bundles. Among the herbaceous Dicotyledons in- 

 crease in thickness is also the rule ; for these plants having annual 

 stems require that sufficient phloem and xylem should be made 

 for purposes of nutrition before the bundles are closed, and that the 

 mechanically supporting tissue should be sufficiently strengthened 

 to bear the increasing strains as the plant grows larger ; and hence 

 a cambium ring is necessary for nutrition and mechanical support. 



The few Dicotyledons mentioned above live under special 

 conditions ; — Hippuris, Myriophyllum, being water plants, require 

 little xylem or supporting tissue ; and hence continual additions 

 to the xylem are not needed, and so the bundles are closed. Again 

 the Saururece and the species of the Ranunculacece (viz. Caltha 

 palustris and species of Ranunculus) are marsh plants, which do 

 not attain to any great size and whose habit is very similar to 

 that of water plants ; and hence these do not require the continual 

 addition of xylem and phloem to their vascular bundles. 



The Gucurbitaceod may roughly be divided into (1) herbaceous 

 climbers with annual stems and (2) woody perennial climbers. 

 The former on the one hand have the stem strengthened by a ring 

 of sclerenchymatous tissue situated in the cortical tissue between 

 the epidermis and the vascular bundles. The support derived 

 from this sclerenchyma and the xylem which is formed before the 

 bundles are closed, together with that which is derived from the 

 external object upon which the plant climbs are sufficient; and 

 hence additional xylem is not required for purposes of support, 

 the plant only requiring that the amount necessary for nutritive 

 purposes should be made before the bundle is closed. The latter 

 on the other hand have no ring of sclerenchyma, and derive their 

 support from the xylem and external objects; but since these 

 plants are perennial, and continually make fresh leaves and 

 branches, they require the constant addition of new xylem and 

 phloem to their stems ; and from this fact we have the reason why 

 a cambium ring is present. 



The explanation why an additional layer of phloem on the 

 inside of the xylem is needed by the members of this order must 

 again be sought for in the special conditions under which they 



