THE LABIATE FAMILY 



105 



a ^ t a 



obtained by the small, nearly linear leaves of the specimens 



growing uncovered, is more than sufficient for their growth. 



Further, -those in shady and moist places will, if plucked, fade 



much sooner than the other kind. Why? Since the place where 



they stood is always moist, they need not be economical with 



water and their leaves are, therefore, large and tender. They 



lack the various means of checking 



the evaporation of water, such as a 



thick epidermis, a small surface, etc. 



(Contrast it with Cactus, p. 57.) 



The same will be found, \i plants 



growing on a rich and a poor soil are 



compared. The difference in this 



case is, however, caused principally 



by the quantity of food the plants 



are able to extract from the soil; 



hence the root-system of those grow- 

 ing in the rich soil will be found to 



l)e much larger than that of plants 



which grow in a poor soil. 



2. The Stem has not only to 



bear its own weight and that of the 



branches with their leaves, but it must also be able to resist 



the bending, twisting, and breaking in- 

 fluences of the wind. If the stem is 

 bent by a rush of w^ind to one side, the 

 parts on that side of the stem towards 

 which it is bent will be pressed together, 

 whereas the other side will be stretched 

 by the tension exerted on it. The middle 

 part will naturally suffer least (fig. 99). 

 Therefore the sides of the stem should be 

 Fig. lOo.-Transverse section strongest. Now, if the stem of the plant 



is cut across (fig. 100), it will at once 

 be seen that this is really the case. 

 There are four bundles of strong fibres 



at the four corners of the stem which thus becomes quadrangular. 



i t 



Fig-, 99. — Transverse section 



tliroug'h a cylinder, straight 



(with dotted lines) and bent (with 



thick lines). In the straight 



cylinder all edges are equally 



long'; in the bent one the inner 



edge (a) is shortened and the 



outer one {a') lengthened. 



of the stem of a Labiate plant, 



Lamium album (40 times 



enlarged). 



