砂和砂石地基底面宜铺设在同一标高上,如深度不同时基底面应挖成阶梯或斜坡形搭接,搭接处应注意压实,施工应按先浅后深的顺序进行。
参考答案:错
脊髓麻醉时对心血管系统的影响主要是由于()
A.血容量的改变
B.节后交感神经阻滞
C.节前交感神经阻滞
D.体位改变
E.禁食水的影响
About fifty years ago, plant physiologists set out to grow roots by themselves in solutions in laboratory flasks. The scientists found that the nutrition of isolated roots was quite simple. They required sugar and the usual minerals and vitamins. However, they did not require organic nitrogen compounds. These roots got along fine on mineral inorganic nitrogen. Roots are capable of making their own proteins and other organic compounds. These activities by roots require energy, of course. The process of respiration uses sugar to make the high energy compound ATP, which drives the biochemical reactions. Respiration also requires oxygen. Highly active roots require a good deal of oxygen.The study of isolated roots has provided an understanding of the relationship between shoots and roots in intact plants. The leaves of the shoots provide the roots with sugar and vitamins, and the roots provide the shoots with water and minerals. In addition, roots can provide the shoots with organic nitrogen compounds. This comes in handy for the growth of buds in the early spring when leaves are not yet functioning. Once leaves begin photosynthesizing, they produce protein, but only mature leaves can "export" protein to the rest of the plant in the form of amino acids.
The scientists found what the isolated roots need is________.
A.quite natural
B.sugar, minerals and vitamins
C.some rare vitamins
D.organic nitrogen compounds