CHAPTER V. — THE BRANCHING OF ORGANS. 47 



infrequently secrete a glutinous or odorous secretion. Such 

 hairs are called grandular. They also may be unicellular, 

 as in the Sage, Fig. 143, or multicellular, as in the House 

 Geranium, Fig. 155. Other forms of hairs are represented in 

 Figs. 161 and 162, and 167, which represent respectively hairs 

 of Heliotropum tenellum, Heliotropom convolvulaceum, and 

 Cassandra calyculata. 



Practical Exercises. 



By means of your magnifying lens examine the following plants, or parts of 

 plants, and describe and draw the shapes of the hairs : The Garden Verbena, or 

 one of the wild Vervains ; the garden Aster ; the young leaves and branches of 

 the house Ivy ; the stems and leaves of Erigeron Canadense ; the leaves of the 

 common Mullein ; the prickles of the Raspberry and Blackberry ; the scales or 

 chaff on the stipes of Aspidium marginale : the young leaves of the Hickory and 

 Walnut ; the leaves and flower-heads of the Pasture Thistle ; the young leaves 

 of the Common Hazel and of the Witch hazel ; the under surfaces of the leaves 

 of Shepherdia ; the leaves of Spanish Moss, and the spore-cases on the leaves of 

 the common Polypodium. Ascertain the probable functions of the hairs in 

 each instance, and state your conclusions. 



CHAPTER V.— THE BRANCHING OF ORGANS. 



Any organ of the plant-body may branch, and the branching 

 is always according to one or other of two general types, the 

 Dichotomons or the Monopodial. In the former mode the branch- 

 ing takes place by forking, or by the repeated division of the 

 apex of the organ into two equal portions, as illustrated in Fig. 

 144. Three different varieties of this mode are observed. 



(1) The Forked Dichotomy, in which the branches develop 

 equally, as in Fig. 169 : 



(2) The Helicoid Dichotomy, in which the branch on one 

 side is invariably suppressed, or less strongly developed than the 

 other, as illustrated in Fig. 170 ; and 



(3) The Scorpioid Dichotomy, in which a branch is sup- 

 pressed, or but partially developed, first on one side and then 

 on the other, as in Fig. 171. 



This plan and its various modifications are more com- 

 monly seen in flowerless than in flowering plants. It is the 



