The Greenhouse and Frame 217 
up. By tapping the pots this can be determined to a great 
extent. With a little experience one can detect the various 
rings of the pot, and thus ascertain, if water is required, if 
at once or later on. Again, by rubbing the forefinger on 
the top soil it is easily ascertained whether it is wet or 
dry. On the other hand, if too much water is given, it 
fills up the spaces between the particles of soil, and air 
cannot get to the roots of the plants nor into the soil to 
allow the little organisms (bacteria) to do their work of 
changing the manure, &c., into liquid plant food. Should 
this state continue it will become stagnant in the soil, and 
the plants will certainly die. 
Of course when the fundamental principles of watering 
are grasped it is only a matter of experience with close 
observation. 
Some plants are adapted for growing under water, or 
partly under water, such as Nymphaeas, or some of the 
Ranunculacese, &c. These plants have special adaptations 
for breathing, such as large air spaces in stems, leaves, 
stalks, &c. Further, we have plants which require very 
little water and which have special adaptations for con- 
serving the water at times, such as the Cacti, &c., with 
their thick, fleshy stems. 
The. moisture required in the greenhouse itself varies 
to a great extent according to the season and weather, and 
according to the kind of plants growing in it. During 
winter it is advisable to keep the house as dry as possible ; 
otherwise, with a humid atmosphere, dull weather, and very 
little growth going on, the plants are liable to be attacked 
by disease, especially the “damping off” disease (Pythium). 
But in late spring and summer, with warm days, bright 
