DEFINITION AND CLASSIFICATION OF STEMS. 653 



flattened shoots, which have been fully described on p. 333. The Horsetails (Equi- 

 setacese) belong also to this group, and in one group of these (Equisetum arvense, 

 E. Tehnateja) the division of labour is similar to that in the Colt's-foot. The first 

 pale shoots which emerge from the ground are terminated by a spike of sporangia- 

 bearing scales, and not until later, after the spores have been scattered by the wind 

 and the pale primary shoots have withered, do the summer shoots appear whose 

 stems develop green tissue in the cortex. 



The inner structure of the green scaly stems, whose duty is to manufacture 

 organic materials, agrees essentially with that of the foliage-stem. In these plants, 

 indeed, the functions have only been transposed in this way, viz. that normal green 

 leaves are not produced, but only small colourless scales, whilst the work usually 

 allotted to the leaves has been assumed by the cortex. Green scaly stems are just 

 as much exposed to wind and sunlight as leafy stems are; they must, like them, 

 direct and establish themselves in accordance with the particular conditions of their 

 habitat and offer the same resistance to the wind; they must be just as elastic and 

 flexible, and consequently present a similar arrangement of their tissues rendering 

 it possible for them to maintain the favourable position once assumed. The sub- 

 terranean scaly stems have no need of such contrivances; no winds press against 

 them and their tissue does not require to be strengthened against bending. The 

 stems of Balanophoreae require only a slight elasticity, the part which rises above 

 the ground is relatively very thick and almost reminds one of the stalks of the 

 cap-fungi. Many of these scale-leaf stems under the ground or rising only a little 

 above it, are very brittle, and when stems of Dentaria, embedded in the humus of 

 the forest soil, are dug up, the greatest care must be taken to prevent their break- 

 ing. The same is true of underground tubers and bulbs; they need none of those 

 contrivances by means of which a definite position with regard to the light, or a 

 great capacity of resistance to wind is obtained. Protective measures against 

 excessive transpiration are likewise unnecessary, and this accounts for the lack of 

 cuticle to the epidermal cells, and for the absence of hair-like structures and var- 

 nish-like coatings. When dry, tough scales occur as envelopes to bulbs, they are 

 probably of a protective nature — not against transpiration or over-illumination, but 

 against subterranean animals which might come and nibble them for their food- 

 reserves. 



These subterranean shoots excavate their own bed by the pressure which their 

 turgescent tissues exert on the surrounding earth during growth. Growing bulbs 

 and tubers in this way widen out a bed, often of considerable size, and the pressure 

 exerted is so great that the loose earth in their neighbourhood becomes compressed, 

 and sometimes transformed into hard cakes. It has already been mentioned that 

 not only stiff soil but even bits of wood and other objects may be bored through by 

 the stiff, pointed scale-leaves of the Couch-grass. A most important function falling 

 to the lot of underground shoots, and especially to tubers and bulbs, is the storage 

 of reserve materials. These are manufactured during the summer by the green 

 tissues in the sunlight above ground and are then conducted down into the 



