CHAP. IV. FUNCTION OF NUTRITION. 235 



vine, sycamore, and many other trees be cut off at this 

 period^ the sap often flows with sufficient rapidity to 

 fill a bottle in a few hours. As the summer advances 

 this action gradually diminishes ; but in the autumn it 

 is again partially renewed. 



(232.) Autumnal Development. The buds formed 

 in the axils of the leaves of many plants have attained 

 by autumn a sufficient size to attract the sap towards 

 them, and then they undergo a partial development, 

 which however is soon checked on the approach of 

 winter. In a few cases, as in the Lombardy poplar, this 

 autumnal development is sufficient to furnish the ex- 

 tremities of some branches with leaves which remain 

 for some time after the older leaves have fallen. This 

 always takes place in mulberry trees in those countries 

 where they are stripped for the purpose of feeding silk- 

 worms. The buds then become the centres of attraction 

 to the rising sap, and soon developing furnish the trees 

 with fresh leaves which replace those that have been 

 removed. Such a tree lives as it were two years in 

 one, but is always proportionably stunted and injured in 

 its growth. 



(233.) Nutrition of Cryptogamic Plants. The 

 higher tribes of cryptogamic plants possess true roots 

 and leaves ; and we may suppose their function of nu- 

 trition to be carried on in a way which differs little 

 from that in which it proceeds among phanerogamic 

 species. But the manner in which the lower tribes 

 whose nutritive organs are not distinguishable into roots 

 and leaves Complete the function is in great obscurity, 

 and few attempts have hitherto been made to elucidate 

 the subject. 



(234.) Parasitic Plants. There are certain plants 

 which are without the means of providing nutriment 

 for themselves or of elaborating the crude sap into 

 proper juice but obtain their nourishment immediately 

 from other plants to which they attach themselves, 

 and whose juices they absorb. Such plants are true 

 " Parasites." They are distinguished from " Epi- 



