512 DEPENDENCE OF PLANT FORM ON SOIL AND CLIMATE. 



have a rounded petiole and a disc-shaped lamina. These characters are always 

 present whether the seed which produced the plant germinates in a deep lake 

 or in the mud of a marshy meadow. In the marshy meadow the scale-leaves 

 remain short, and the walls of their epidermal cells thicken in a remarkable 

 way; the petioles of the aerial foliage-leaves become about a span long, and, in 

 order to increase their resistance to bending, a strong layer of bast arises, the 

 thickness of these bast-layers amounting to 0'17 mm. The walls of the epidermal 

 cells are thickened, 6-9 layers of collenchymatous cells are formed under the 

 epidermis with walls 007 mm. thick, and the air-spaces in the centre of the leaf- 

 stalk are much narrowed. But if this species of Water-lily grows under water, 

 the scale-leaves elongate into long and flaccid ribbons, and the petioles of the 

 foliage-leaves continue to grow until their blades are raised to the surface of the 

 water. According to its depth they attain a length of 30, 40, 50-100 cm. 

 Resistance to bending is but little required by the petioles, which are surrounded 

 by water, and the bast is therefore only slightly developed. The strings of 

 bast which traverse the leaf -stalk are only Oil mm. thick, the walls of the 

 epidermal cells are only half as thick as in the aerial leaves, only 3-5 layers of 

 collenchyma are developed below the epidermis and the air-spaces in the centre 

 of the leaf-stalk have a diameter of over half a millimetre. These petioles are 

 consequently flexible, and cannot support the leaf -blade if taken out of the water. 

 The general form of the scale- and foliage-leaves, the segmentation of the latter 

 into petiole and blade, the configuration of the blade and the distribution of 

 the bundles in it are all the result of internal forces due to the specific constitution 

 of the protoplasm; but the thickness of the epidermal cells, the strength of the 

 mechanical tissue, and the length of the leaf-stalk, are determined by the depth 

 of the water- covering. The same thing is seen in the flowers; their structure 

 depends upon the specific constitution of the protoplasm, but the size of the 

 petals is determined by the temperature of the water. 



The Meadow-grass Poa annua has a rapid growth; its haulms and leaf -sheaths 

 are round, the leaf -lamina is traversed by seven strands, the lower branches of the 

 inflorescence are single or paired but never whorled, and the spikelets of the 

 panicle are much compressed and egg-shaped in outline. These characters are 

 unalterable and are observed in Poa annua under all conditions. But when the 

 haulms growing in the gardens in the plain project beyond the short upmost leaf 

 the spikelets become 6-7 -flowered, and have a pale green colour. When the plants 

 become perennial in alpine regions the haulms bend towards the ground and remain 

 so short that they do not reach above the highest foliage-leaf; the spikelets 

 develop only 3-4 flowers, and their glumes are dark violet on the surface and 

 brownish-yellow at the edge; thus these modifications are in relation to peculiarities 

 of situation (in the plain and alpine regions) as effect to cause, and are to be 

 ascribed to the influences of heat, light, and moisture, which act in various ways 

 according to the situation. 



These alterations are always to the advantage of the plant. They make the 



