32 



THE GARDENER'S ASSISTANT. 



to be attributable to the combined effect of heat 

 and light. As the temperature increases, and 

 the light gains in intensity, the flowers open, 

 activity or turgescence being most apparent 

 in the dorsal or outer surface of the petals. 

 Then, as the temperature falls and the light 

 diminishes, the energy is most displayed on 

 the inner surface, causing them to close over 

 and shelter the interior of the flower from 

 excessive nocturnal radiation. 



The influence of light is further shown by 

 the way in which the leaves and flowers move 

 and turn so as to expose themselves to it ; such 

 movements are called " heliotropic ". On the 

 contrary, roots have a tendency to avoid the 

 light, and in the case of ivy growing on a wall 

 we have a marked illustration of the leaves 

 turning towards the light, whilst the rootlets 

 protruding from the stem grow in the opposite 

 direction. 



The " heliotropic " movement in the case of 

 leaves of course favours the process of nutrition 

 or assimilation, but the actual growth which 

 results from that assimilation is favoured by 

 darkness. A plant grows more vigorously in 

 the dark, other conditions being equal, than in 

 the light. Roots show a marked tendency to 

 move and grow in the direction where water is 

 to be found, a phenomenon which is indicated 

 rather than explained by calling it " hydro- 

 tropism", [m. t. m.] 



CHAPTER III. 



THE RADICLE AND THE ROOT. 



Nature and Movements — Root-hairs — The Root-cap 

 — Root Action — Various Forms of Roots. 



As a general rule, subject to very few ex- 

 ceptions, all the higher green plants have roots, 

 and even many of the Thallophytes are pro- 

 vided with cells, which are root-like in appear- 

 ance and oflice. In the higher seed-bearing; 

 plants the root is the first member that be- 

 comes visible when the seed-shell cracks open to 

 liberate the growing embryo or seedling plant. 

 Almost any germinating seed will illustrate the 

 chief features of the root; take Penstemon 

 as a convenient example (fig. 28), or the gourd 

 (fig. 34). The radicle, or primary root, as it first 

 emerges is a mere thread tapering, or a little 

 pointed, at the free end. That free end has an 

 invincible tendency to turn down towards the 

 earth. If so placed that it must perforce go up- 

 wards, it will, as soon as the resistance is over- 

 come or evaded, turn downwards. The down- 



ward direction of the root is, of course, obvious 

 to every observer. Not so readily seen is the 

 gyratory or twisting movement that takes place 

 at the tip of the growing root, nevertheless it is 

 as general a feature of the root as is its down- 

 ward course. How the 

 roots descend, and how 

 they gyrate, must be left 

 to the physiologist to 

 ascertain. It is of more 

 concern to the gardener 

 to know why they do so, 

 and as to this point 

 there is not much diffi- 

 culty. They worm their 

 way amid the particles of 

 soil in order to extract 

 from them their food 

 supplies. To appreciate 

 let us glance for a moment at their struc- 



Fig. 28. — 1. Seedling of Penste- 

 mon with the long absorptive 

 cells of its root (" Root-hairs") 

 with sand attached. 2. The 

 same seedling: the sand re- 

 moved by washing. 



tb 



ture, remembering that we are concerned at 

 present with the root in its simplest and 

 youngest state. It is now a delicate thread 

 destitute of green matter, and unprovided with 

 leaves, leaf-scales, or buds. It cannot, there- 

 fore, do the same sort of work that the leaf 

 does. An examination with a pocket lens, 

 which no gardener should be without, will, in 



Fig. 29.— Root-tip of Penstemon with Root-hairs penetrating 

 between the particles of soil ; X 10. 



many cases, show, just above the extreme tip 

 of the root, a fringe of the finest possible white 

 glistening threads. These are the root -hairs. 

 When food supplies or moisture is abundant, 

 and easy to procure, then these root-hairs are 

 proportionally numerous, and, as in the case of 

 vine roots in a well-made border, may be seen 

 investing the fragments of manure or bone as 

 it were with a miniature cobweb (figs. 29, 30). 



