220 



HUTCHINSON'S POPULAR BOTANY 



on the size of plants of the same genus, and even of the same species, 

 is in some cases extremely curious. Heat is a great stimulus to growth, 

 and many plants which attain to the dignity of trees in tropical and 

 sub-tropical countries will degenerate into mere shrubs when grown in 

 more temperate regions. Speaking generally, the farther north we go 

 the more stunted is the vegetation : but the 

 difference observable in plants of the same 

 species even when growing in neighbouring 

 countries is frequently very marked. A striking 

 illustration of the above facts is afforded by 

 the Willows (Salix). some of which in this 

 country are timber-trees of considerable dimen- 

 sions, while in the Arctic regions their repre- 

 sentatives seldom attain the height of nine 

 inches ! Salix herbacea, myrtilloides, pyrenaica, 

 and reticulata, all species found in the ice- 

 regions of North America, arrive at maturity 

 and bear their flower-catkins when they are 

 scarcely six inches above the ground ! Some 

 of these small trailing forms we have on our 

 own moors and heaths. 



To come back to the stem. The points on 

 the stem where leaves are given off and buds 

 formed are called nodes ; the spaces between, 

 internodes. Recently Professor L. Celakovsky, 

 of Prague, has propounded a new theory re- 

 specting the building up of the stem. As just 

 stated, the view formerly held by botanists was 

 that the internode consisted of all that section 

 of the stem lying between two nodes, but in 

 Celakovsky's opinion this view requires some 

 qualification when applied to dicotyledons. Ac- 

 cording to a notice of this theory by W. C. 

 W[orsdell] in the Xenv Phytologist, the Bohemian 

 botanist divides stems into two classes holo- 

 cyclic and mericydic. Holocyclic stems consist 

 of a series of joints or internodes placed one 

 above another, each occupying the entire 

 diameter of the axis and terminating at the node in a leaf. As each 

 leaf arises from that portion of the apex which becomes the stem-joint 

 to which it belongs, we may regard, he says, the leaf along with the 

 latter as a morphological unity, and term it a Sprossglied (shoot segment). 

 The entire monocotyledonous embryo (apart from the root) represents a 

 first such Sprossglied, the hypocotyl being its holocyclic Stengelglied (steni- 



FIG. 275. " BUSH- ROPE.' 



Portion of stem of a Bauhinia. 



