CiTIAP^PKR VIII 



AN INDIVIDUAL PLANT IN ALL OF ITS RELATIONS 



For the 2)ui'pose of sninniariziiii; the genci'al lifc-i'ehi- 

 tions detailed in tlie 2>rccediiiii,' chapters, it will lie. useful to 

 ap}>ly them iu the ease of a single jilaut. 'J'akiug a eom- 

 inou seed-plant as an illusti-ath.ui, and foUowiug its liistory 

 from Uw germination of the seed, ceiia.in general fa.ets 

 become evident, in its relations to the extei'ual wdrhl. 



'.)4. Germination of the seed. — 'I'he most. ol)\ ious needs of 

 the se(?d I'or germinatidu are certain amounts (d' nmisture 

 and heat. In oi'der to secure these to the l)est a.dvanla.ge, 

 the seed is usually very delinitelv related to the soil, either 

 upon it a.ml covered hy moisture and heat-reta.iiung dehris, 

 or emliedded in it. Along with the denia.ml for heal and 

 moisture is oiu' for air (su])]ilying (ixygcn). winch is essen- 

 tial to lile. The relation which germinaiing seeds need, 

 thercfoi'e, is o\w Avhicli not only se(uires moisture and heat 

 advantageously, hut permits a free cir(ndation of air. 



D.'i. Direction of the root. — The lirst part of the \(ning 

 2)lantlet to emerge from the seed is the tip of the axis 

 which is III devehiji the root system. If at oiu'c a.]i]iea.rs to 

 he very scnsiti\'e to the cai'th inlluence (//('c//-c/i/,s///) and 

 to moisture inllueiic(^ (Iii/i/ro/rd/iisiii), I'nr \vluite\er the 

 diriMition of eiiHTgenee from the seed, a cui'vature is de\"el- 

 oped which dii'eids tin; tip towa.rds and linally into the soil 

 (see Fig. 14.'!). When tlu^ soil is penetrated the iiriniary 

 root may coidinue to grow vigorously downward, showing 

 a strong geotropic tcmlency, and fornung what is known 

 as the tap-root, from which lateral roots arise^ which are 

 i:J8 



